Under Windows this is an option in the AMD control panel. Not sure how to enable it.
Specifically, this is for a non-native Proton game with FSR support built in. I am not using GE as I need the latest code in Proton experimental to run this game.
Yeah, don't do that. You're asking for the game to shut off at best. They probably won't have the anticheat configured to outright ban you for that but, as a general rule: do not tamper with multiplayer anti-cheat protected games.
My recommendation would be to go to their discord and request the option.
EDIT: I will say this is overly cautious advice, I've never seen Crytek in general or any other developer ban people for using render alternating software that wasn't specifically designed to be a cheat.
The game doesn't even realise gamescope is there. That's the beauty of it. As the game sees it, it's just a regular monitor with the specs you gave it via commands