diff --git a/AllenOnline/CMakeLists.txt b/AllenOnline/CMakeLists.txt
index 50dbf1e90858e7aa23356c106c53182c9bec601f..695783918ed7d326f3be13cc286962544ea429d2 100644
--- a/AllenOnline/CMakeLists.txt
+++ b/AllenOnline/CMakeLists.txt
@@ -19,7 +19,6 @@ gaudi_add_library(AllenOnlineLib
         src/TransposeMEP.cpp
     LINK PUBLIC
         Gaudi::GaudiKernel
-        cppgsl::cppgsl
         Online::EventBuildingLib
         Allen::AllenLib
 )
@@ -50,7 +49,6 @@ gaudi_add_module(AllenOnlineComp
         Online::OnlineBase
         Online::RPC
         Online::dim
-        cppgsl::cppgsl
         rt
 )
 
diff --git a/AllenOnline/application/MPISend.cpp b/AllenOnline/application/MPISend.cpp
index 8a1dc9cbd37ad8c8b3676a67a3498e83e5c2c8c5..46f017f3ffe34b2c2e7b3e46ed7bf25cfa561975 100644
--- a/AllenOnline/application/MPISend.cpp
+++ b/AllenOnline/application/MPISend.cpp
@@ -77,13 +77,13 @@ int main( int argc, char* argv[] ) {
   std::vector<MPI_Request> requests( window_size );
 
   // Read all files in connections
-  std::vector<std::tuple<EB::MEP const*, gsl::span<char const>>> meps;
+  std::vector<std::tuple<EB::MEP const*, std::span<char const>>> meps;
 
   std::cout << MPI::rank_str() << "Reading "
             << ( number_of_meps != 0 ? std::to_string( number_of_meps ) : std::string{"all"} ) << " meps from files\n";
 
   std::vector<char>     data;
-  gsl::span<char const> mep_span;
+  std::span<char const> mep_span;
   size_t                n_meps_read = 0;
 
   std::optional<unsigned> packing_factor = std::nullopt;
@@ -115,7 +115,7 @@ int main( int argc, char* argv[] ) {
         std::copy_n( mep_span.data(), mep_span.size(), contents );
         ++n_meps_read;
 
-        meps.emplace_back( mep, gsl::span<char>{contents, mep_span.size()} );
+        meps.emplace_back( mep, std::span<char>{contents, mep_span.size()} );
       }
       if ( n_meps_read >= number_of_meps && number_of_meps != 0 ) {
         input.close();
diff --git a/AllenOnline/application/bench_mep_offsets.cpp b/AllenOnline/application/bench_mep_offsets.cpp
index 3ef6559c449de97de057ed039293977716e4971f..62460c68a91a8048204f5aee408f5e71262666ed 100644
--- a/AllenOnline/application/bench_mep_offsets.cpp
+++ b/AllenOnline/application/bench_mep_offsets.cpp
@@ -81,7 +81,7 @@ std::vector<char> contiguous_mfps( Allen::Slice const& mep_data ) {
   auto const&  mfps = mep_data.fragments;
   vector<char> mep_fragments( mep_data.fragments_mem_size, '\0' );
   char*        destination = &mep_fragments[0];
-  for ( gsl::span<char const> mfp : mfps ) {
+  for ( std::span<char const> mfp : mfps ) {
     ::memcpy( destination, mfp.data(), mfp.size_bytes() );
     destination += mfp.size_bytes();
   }
diff --git a/AllenOnline/include/AllenOnline/ReadMEP.h b/AllenOnline/include/AllenOnline/ReadMEP.h
index df6def1d5fb3f6e3ce6d0537e7339c2df0dc4570..f43eb1f2dc740e7c130c33c8d48294e737fb0b52 100644
--- a/AllenOnline/include/AllenOnline/ReadMEP.h
+++ b/AllenOnline/include/AllenOnline/ReadMEP.h
@@ -4,15 +4,14 @@
 #pragma once
 
 #include <iostream>
+#include <span>
 #include <vector>
 
-#include <gsl/gsl>
-
 #include <EventBuilding/MEP_tools.hpp>
 #include <GaudiKernel/MsgStream.h>
 #include <MDF/StreamDescriptor.h>
 
 namespace MEP {
-  std::tuple<bool, bool, EB::MEP const*, unsigned, gsl::span<char const>>
+  std::tuple<bool, bool, EB::MEP const*, unsigned, std::span<char const>>
   read_mep( LHCb::StreamDescriptor::Access& input, std::vector<char>& buffer, MsgStream& out_stream );
 }
diff --git a/AllenOnline/include/AllenOnline/TransposeMEP.h b/AllenOnline/include/AllenOnline/TransposeMEP.h
index e89880eb2542329c2399474bc355a9ebff245a77..db90dc8a06f9e5123994321739deaf89a703190d 100644
--- a/AllenOnline/include/AllenOnline/TransposeMEP.h
+++ b/AllenOnline/include/AllenOnline/TransposeMEP.h
@@ -61,7 +61,7 @@ namespace MEP {
 
   struct Slice {
     EB::MEP const*        mep = nullptr;
-    gsl::span<char const> mep_data;
+    std::span<char const> mep_data;
     unsigned              packing_factor = 0u;
     Blocks                blocks;
     SourceOffsets         offsets;
diff --git a/AllenOnline/src/AllenApplication.cpp b/AllenOnline/src/AllenApplication.cpp
index a78430856d4be1d7122dee61ecd33146f95d4f8e..9fe43b54b036cddf537742911275415f072d909b 100644
--- a/AllenOnline/src/AllenApplication.cpp
+++ b/AllenOnline/src/AllenApplication.cpp
@@ -302,7 +302,7 @@ void AllenApplication::allenLoop( std::string_view config, std::string_view conf
   }
 }
 
-void AllenApplication::update( gsl::span<unsigned const> odin_data ) {
+void AllenApplication::update( std::span<unsigned const> odin_data ) {
   LHCb::ODIN odin{odin_data};
   auto const run = odin.runNumber();
 
diff --git a/AllenOnline/src/AllenApplication.h b/AllenOnline/src/AllenApplication.h
index 4c0f6c3bb95fdfefb85f2f6e36844cecf567a303..dd0d68e4701015d51b3e474d823149dbc4f5e20c 100644
--- a/AllenOnline/src/AllenApplication.h
+++ b/AllenOnline/src/AllenApplication.h
@@ -66,7 +66,7 @@ public:
   bool initMPI();
 
   // Updater
-  void update( gsl::span<unsigned const> odin_data ) override;
+  void update( std::span<unsigned const> odin_data ) override;
 
   void registerConsumer( std::string const& id, std::unique_ptr<Allen::NonEventData::Consumer> c ) override;
 
diff --git a/AllenOnline/src/MBMOutput.cpp b/AllenOnline/src/MBMOutput.cpp
old mode 100755
new mode 100644
index 579b5abc5738d978ba231df2e4cf3e4d3c776dfd..aa943d3f9dae0fcc85af53464154c0b6fdbb39eb
--- a/AllenOnline/src/MBMOutput.cpp
+++ b/AllenOnline/src/MBMOutput.cpp
@@ -38,7 +38,7 @@ namespace Allen {
     StatusCode start() override;
 
   protected:
-    gsl::span<char> buffer( size_t thread_id, size_t buffer_size, size_t n_events ) override;
+    std::span<char> buffer( size_t thread_id, size_t buffer_size, size_t n_events ) override;
 
     bool write_buffer( size_t thread_id ) override;
 
@@ -49,7 +49,7 @@ namespace Allen {
   private:
     struct MBMSlot {
       BMID            id = MBM_INV_DESC;
-      gsl::span<char> buffer;
+      std::span<char> buffer;
       size_t          n_events = 0;
     };
 
@@ -94,7 +94,7 @@ Allen::MBMOutput::MBMOutput( std::string name, ISvcLocator* loc ) : Service{name
 
 int Allen::MBMOutput::spaceCallback( void* /* param */ ) { return MBM_NORMAL; }
 
-gsl::span<char> Allen::MBMOutput::buffer( size_t thread_id, size_t buffer_size, size_t n_events ) {
+std::span<char> Allen::MBMOutput::buffer( size_t thread_id, size_t buffer_size, size_t n_events ) {
   if ( m_cancelled ) { return {}; }
 
   auto& slot    = m_buffers[thread_id];
@@ -187,7 +187,7 @@ StatusCode Allen::MBMOutput::start() {
     } else {
       bmID = mbm_connect( m_buffers[0].id, m_processName.c_str(), m_partitionID );
     }
-    m_buffers.emplace_back( MBMSlot{bmID, gsl::span<char>{}, 0u} );
+    m_buffers.emplace_back( MBMSlot{bmID, std::span<char>{}, 0u} );
 
     if ( bmID == MBM_INV_DESC ) {
       error() << "MBMOutput: failed to connect to MBM buffer for thread " << id << " " << connection() << endmsg;
diff --git a/AllenOnline/src/MEPProvider.cpp b/AllenOnline/src/MEPProvider.cpp
old mode 100755
new mode 100644
index 9ab678f46a18e5abefe2229641a850517399e54b..cb51931334ce27936825096fec5c442d25993b2d
--- a/AllenOnline/src/MEPProvider.cpp
+++ b/AllenOnline/src/MEPProvider.cpp
@@ -131,7 +131,7 @@ std::tuple<bool, bool, bool, size_t, size_t, std::any> MEPProvider::get_slice( s
     if ( !m_read_error && has_transposed() && ( !timeout || ( timeout && !timed_out ) ) ) {
       transposed = get_transposed();
       if ( transposed->n_transposed && *( transposed->n_transposed ) > 0 ) {
-        odin_span = gsl::span<unsigned const>{m_odins[transposed->slice_index]};
+        odin_span = std::span<unsigned const>{m_odins[transposed->slice_index]};
       }
     }
   }
@@ -227,7 +227,7 @@ void MEPProvider::slice_free( size_t slice_index ) {
   }
 }
 
-void MEPProvider::event_sizes( size_t const slice_index, gsl::span<unsigned int const> const selected_events,
+void MEPProvider::event_sizes( size_t const slice_index, std::span<unsigned int const> const selected_events,
                                std::vector<size_t>& sizes ) const {
   int    i_buffer       = 0;
   size_t interval_start = 0, interval_end = 0;
@@ -243,7 +243,7 @@ void MEPProvider::event_sizes( size_t const slice_index, gsl::span<unsigned int
   }
 }
 
-void MEPProvider::copy_banks( size_t const slice_index, unsigned int const event, gsl::span<char> buffer ) const {
+void MEPProvider::copy_banks( size_t const slice_index, unsigned int const event, std::span<char> buffer ) const {
   auto [i_buffer, interval_start, interval_end] = m_slice_to_buffer[slice_index];
   const auto mep_event                          = interval_start + event;
 
@@ -1228,7 +1228,7 @@ void MEPProvider::mpi_read() {
         break;
       }
 
-      slice.mep_data = gsl::span{contents, static_cast<events_size>( slice.slice_size )};
+      slice.mep_data = std::span{contents, static_cast<events_size>( slice.slice_size )};
     }
 
     // Number of full-size (MPI::mdf_chunk_size) messages
@@ -1264,7 +1264,7 @@ void MEPProvider::mpi_read() {
     MPI_Waitall( n_sends, requests.data(), MPI_STATUSES_IGNORE );
 
     slice.mep      = reinterpret_cast<EB::MEP const*>( contents );
-    slice.mep_data = gsl::span{contents, static_cast<events_size>( mep_size )};
+    slice.mep_data = std::span{contents, static_cast<events_size>( mep_size )};
 
     auto const* mep      = slice.mep;
     auto const* mfp      = mep->at( 0 );
diff --git a/AllenOnline/src/MEPProvider.h b/AllenOnline/src/MEPProvider.h
index e127dc4da6a4a2717acbe9bfb80330a052df5448..5bf58655b01029f9e0501524d58b9bd54fb59d52 100644
--- a/AllenOnline/src/MEPProvider.h
+++ b/AllenOnline/src/MEPProvider.h
@@ -142,10 +142,10 @@ public:
 
   bool release_buffers() override;
 
-  void event_sizes( size_t const slice_index, gsl::span<unsigned int const> const selected_events,
+  void event_sizes( size_t const slice_index, std::span<unsigned int const> const selected_events,
                     std::vector<size_t>& sizes ) const override;
 
-  void copy_banks( size_t const slice_index, unsigned int const event, gsl::span<char> buffer ) const override;
+  void copy_banks( size_t const slice_index, unsigned int const event, std::span<char> buffer ) const override;
 
   StatusCode initialize() override;
 
diff --git a/AllenOnline/src/ReadMEP.cpp b/AllenOnline/src/ReadMEP.cpp
index da7334ce5ba16855a4330842c59e580cef6545c5..fb276d2ab6e3af7db767fd5578124bc341b84efa 100644
--- a/AllenOnline/src/ReadMEP.cpp
+++ b/AllenOnline/src/ReadMEP.cpp
@@ -10,10 +10,9 @@
 #include <cstring>
 #include <iostream>
 #include <limits>
+#include <span>
 #include <vector>
 
-#include <gsl/gsl>
-
 #include <Allen/BankTypes.h>
 #include <EventBuilding/MEP_tools.hpp>
 #include <EventBuilding/MFP_tools.hpp>
@@ -30,7 +29,7 @@
  *
  * @return     (eof, success, mep_header, span of mep data)
  */
-std::tuple<bool, bool, EB::MEP const*, unsigned, gsl::span<char const>>
+std::tuple<bool, bool, EB::MEP const*, unsigned, std::span<char const>>
 MEP::read_mep( LHCb::StreamDescriptor::Access& input, std::vector<char>& buffer, MsgStream& out_stream ) {
   // Allocate space for the first few words of the MEP header
   buffer.resize( sizeof( EB::MEP_header ) );
diff --git a/AllenOnline/src/TransposeMEP.cpp b/AllenOnline/src/TransposeMEP.cpp
index cc65b14f97ecccee04ca46b242d7302819c2988d..054fdd304613093ec5faa80338dd34546df38182 100644
--- a/AllenOnline/src/TransposeMEP.cpp
+++ b/AllenOnline/src/TransposeMEP.cpp
@@ -12,7 +12,7 @@
 
 namespace {
   template <typename T>
-  T* event_entries( gsl::span<unsigned int> offsets, unsigned const event ) {
+  T* event_entries( std::span<unsigned int> offsets, unsigned const event ) {
     return reinterpret_cast<T*>( &offsets[0] ) + offsets[event];
   }
 } // namespace
diff --git a/cmake/MooreOnlineDependencies.cmake b/cmake/MooreOnlineDependencies.cmake
index 6406337cd518f26b8ef1601adfc18ff96e697121..bf0011f2ffa476b3e36a0a6c558f81f0af02e88a 100644
--- a/cmake/MooreOnlineDependencies.cmake
+++ b/cmake/MooreOnlineDependencies.cmake
@@ -34,7 +34,6 @@ pkg_check_modules(sodium libsodium REQUIRED IMPORTED_TARGET)
 if(WITH_MooreOnline_PRIVATE_DEPENDENCIES)
     find_package(Python REQUIRED Interpreter)
     find_package(MPI QUIET COMPONENTS C CXX)
-    find_package(cppgsl REQUIRED)
     find_package(Boost REQUIRED program_options)
 
     pkg_check_modules(hwloc QUIET IMPORTED_TARGET hwloc)