Both Chow Yun-fat and Andy Lau did their own card manipulations.
This movie started a new wave of Chinese gambling movies.
The God of Gambler's "card changing" technique is a form of handmucking. The technique used in the film consisted of switching cards from the same deck. However, not all of the steps are shown.
The card game played is 5-card stud poker. It was popular among many high-stakes gamblers in the US from the early 1900s to the early 1960s. While this game is still played privately in the US, it's not played that widely anymore. Also, the Asian & US rules are slightly different.
The card marking technique used in the finale is an advanced form of illuminous marking, using certain chemicals not visible to the naked eye. To see the markings, you need a certain pair of glasses or contact lens. However, as you can see in the final showdown, marking cards doesn't always work.