GTA V takes less than D:OS2 now AFAIK.

That's why I was pointing at the option to make both versions "DLC" so you can switch them on and off, without having to manually dig into files and potentially delete the wrong ones.
If you want one? That's possible. Want the other? Possible. Both? Possible. Just not required.

I doubt GOG has both versions installed with it's installer, does it?
A new ID isn't necessary entirely, but having a way not to DL/install a 30GB deadweight? Yeah, that should have been there. And it wouldn't be too difficult to add rather than having people instruct to delete stuff manually (Or DL all this before deleting it in the first place).