GTA5 on Steam says it requires 105GB. It’s not unreasonable to think that GTA6 would take more than 128GB.

Of course with compression (at the cost of longer load times) and/or lower quality textures, I would bet you could make it fit.

I also seem to recall some disk based games in the past containing a subset of the full game. Although at that point you don’t really “win” by having it on disk of isn’t the full game.

Or you could do multiple disks.