GTA is many things for many different people. For some people, it's a racing game, for others it is about socializing and modding cars, for some it's a dogfighting game, some spend thousands of hours grinding the same set of PvE missions (for some reason), and for many it's a third and first person shooter / warzone simulator.

I totally agree, the same way many games are. But in the context of saying "x is by far most successful y on the planet" I think we have to stop somewhere. Otherwise an open game like GTA V becomes the most successful game (in terms of copies sold) in pretty much every category you can't reasonably stick Minecraft in. I don't think there's much value in that.