This isn't even confirmed by rockstar, is it?
Anyways, as for some of the smaller deals they don't have buttons for, I think they could use the D-pad as some (or even just 1) button... On the PS2 games, the D-pad is just used to control the character, but I think most people use the analog stick, and how many people are gonna use NGC's crazy out of the way, non-analog D-pad to control the character? I dunno if that would make up for all the missing buttons (since L3 and R3 are also used in GTA), but it's a start
Oh and I don't think the games would sell too well on NGC or Xbox either... Unless they add something that makes it really worth owning, because the games will be years old and so many people would have already bought them, and that would probably only leave people who ONLY have an NGC or Xbox (and don't have the games on PC) to buy the games... *shrugs*