Recently, GTA Online introduced the BattlEye anti-cheat system, resulting in gamers playing on the Steam Deck experiencing issues accessing the game. This is because Valve's portable console does not support this tool. Rockstar is already aware of the issue and has even published an official response on their FAQ page. They recommended that players contact Valve support to resolve this problem.
It has been reported that players can access GTA Online on the Steam Deck for about a minute before being kicked from the session. The same issue affects Linux users. According to the author of GamingOnLinux, it is potentially possible to add full support for BattlEye in Proton, the compatibility layer used by the Steam Deck. As for the GTA 5 story mode, it remains available on the Steam Deck.