Well... You need to make sure GTA 5 doesn't find the mods you have, and the easiest is to have 2 game copies.
An alternative- is to know which files are mod files and move them away from your GTA 5 directory when you want to play.

There's also a possibility that someone has made a mod manager of sorts that can do this for you.

If you have the steam version- you can delete files that aren't named something x64y.rpf, and do a steam verification of the games integrity.