Can you stream games?

Can you e.g. Stream GTA V on Twitch and earn money, or do you have to ask the game developer for permission?

Yes you can…

As far as I know there's no real rule on this but developers can still say they don't want it because the image / video is owned by the developer. It is only really banned if there's a confidentiality agreement (NDA) in place and it is violated.

For most developers it is free advertising so it is tolerated. The best examples are Fall Guys, Among Us or this horror game where the name doesn't come to mind.