I have the Rockstar Games Launcher version of Grand Theft Auto V. How can I add it into GOG Galaxy 2.0?
Integrations between GOG Galaxy 2.0 and 3rd party clients are nearly all community driven. You can search for community integrations for a specific 3rd party client in the settings menu. I'm working from home right now, so I can't easily check the client, but I think I already managed to add my own Rockstar games launcher integration to my own install. I might be mistaken on that though.