Hello. Is there any way to stop GOG from starting every time I start a GOG game?

a) It's called Galaxy, not GOG
b) Yes, just direct the shortcuts to the appropriate .exe file in the game install directory.

Will the forum stop starting too?

Basically, look for where the game installed to (i.e C:\Program Files (x86)\GOG Galaxy\Games\Your Game) and open the game folder. Look for the main executable (i.e YourGame.exe) (this might have the game icon) (it will also say 'application' in the 'type' column). Right click this and go to 'send to' and choose 'Desktop (Create Shortcut)'. Now, launch the game from the new shortcut, and you're off!

Uninstall Galaxy, 100% guarantee of success.