I've followed the steps to downgrade but when I go to launch the game, it says cannot find gtav.exe. Any help is appreciated.

verify game files and try using the Autopatcher: https://drive.google.com/open?id=13BanLSa7552Gj_qt22ShnogXcewPblk-

When I do that, the autopatcher says that it cannot find update.rpf but they're in the same folder.

well update.rpf shouldnt be in the main folder. It should be inside the update folder