Grand Theft Auto III and Vice City will both easily fit onto one Gamecube disc each. All you need is 1337 compression skeelz (It can be done on GCN) and MP3 music (can also be done on GCN).
Heck, the PC version of Vice City not only has higher resolution textures than th PS2 version, but it is a mere 2 CD's. Even if you do full install (copying music to hard drive also) it comes down to like 1.55 Gb, and since the GCN would not have high res textures (or as high as the PC version), it would be less than that. Plus, the GCN supports texture compression....
Anyway, both games would fit. If the PC version only takes up 2 CD's, and barely 1.5 Gb after installing the game (with texture compression if your vid. card supports it, and the GCN supports texture compression too), then it could fit on a GCN disc if they put some effort into it.
And if they are lazy, they could just do 2 discs - split the city in half.
Of course, it probably won't come to GCN, and if it does, I'm definately not gonna buy it. I'm picking up my PC Vice City next week.