GTA 5 itself is CPU bound lspdfr and mods rarely touch your CPU unless your on something old or dont have enough cores and no hyperthreads dont count and yes i know how they function.
However GTA 5 is a open world game. With such games RAM is actually the 3rd top thing you need to run this game with good FPS 1) GPU 2) CPU 3) RAM
Yes normal games usually dont use more thant 5-6 gigs of ram. With GTA and other open world games however the amount can double easily depending on your settings.
Not saying your issue is RAM but check your system when running the game to find out how much of each resource your pc uses to run the game than adjust.