The save files are stored in a documents folder somewhere in compatdata instead of your real documents folder for some reason. I got to it by searching "Rockstar Games" in compatdata using KFind, but if I can edit this then I'll put the exact path name.

I tried importing a 100% save I found online for legacy edition, but I'm not sure if raw legacy saves are unsupported if they don't go through Rockstar's one-time transfer service or if it's maybe just because it was for a version that was too far out of date, as I remember seeing in a Steam Community post that legacy edition saves work just fine and they are indeed the same format.