(remove multiplayer, crush audio to mono 32kbps, downscale textures to 256x256, delete cutscenes): ~8 GB . But that game would look and sound like a PlayStation 1 title. It wouldn't be "GTA V" anymore.
Have you encountered a fake "1GB GTA 5" download? Share your story in the comments (but don't share the link!). gta 5 highly compressed 1gb for pc