Windowed to fullscreen help

How do I change the game from windowed mode to Fullscreen mode OUTSIDE the game itself. For some reason, I can’t run Garry’s mod in window mode, and only in Fullscreen. during an update, it threw me back to windowed mode, and the game will crash after saying something about a resoultion error. I know the solution is to get it to fullscreen mode, cause I did it before. but I forgot, and all my attempts to get it to work has failed. is there a way to get the game to fullscreen mode outside the game?

-fullscreen under launch options will allow you to run it full screen and you can set your resolution with -w <width> and -h <height> launch option are under properties for the specific game

thanks