Garfield shown as a kitten crosses the street there is a place he passes by called Lorenzo's Music Store which is a nod to Garfield's original voice actor Lorenzo Music.
The code that Garfield uses to unlock his phone is "061978", the date of the first "Garfield" comic strip (June 19, 1978).
The number on the waste container where Garfield is left behind is the date of the very first Garfield comic strip, "06-19-78" (June 19, 1978).
The scene of Jon bathing Garfield is reminiscent of the September 5, 1978 strip, where Jon is also seen wearing a helmet and gloves while giving him a bath. Likewise, the follow-up comic has Jon dry up Garfield, which results in Garfield being all fluffed up, much like in the movie.
At one point Garfield lands on the front of the car windshield smiling at the driver which is a homage to the vintage Garfield plush toys with suction cups on them that stick to the windows of cars. The gag was also reused from Garfield: Der Film (2004) released 20 years earlier.