Just about everything else one would need to run a casino. Cards, tables, electronic displays, shufflers, felts, chairs, pit stands, and currency handling equipment like plastic drop bags are just some of the things they offer. They also sell maintenance services.
Underlying it all: they hold all the various licenses required for just about any aspect of a casino, including maintenance. As an example, they offer pre-shuffled decks which you might think is no big deal until you learn that there's a huge amount of red tape that goes in to making sure decks are truly random and there's no opportunities for tampering. They charge a premium for this but casinos have run the numbers and found it more cost effective to buy decks that can be put into play immediately.
With respect to the games, believe it or not most table games are protected by patent. GPI owns or has arrangements to resell just about any game out there.