I'm on 8GB (+) ram use with just games like Cities: Skylines on. With GTA5 I was well in the 10gb range before.
I can manage 6gb ram use when just ''working'' on the pc ( browser, IDE's like VS, mail, utilities, etc).
Anything ''free'' is used for caching, I don't see why people think it's pointless, if anything 32gb should be the way to go.
I turn my pc on from standby and my last used games are all cached and load much faster, and that's with just 16gb ram...
8GB ram is enough for GTA 5, but it means a lot will get chucked out you want to keep in, like all the browser windows/tabs and everything you run in the background.