Severely off-centered screen

Whenever I start up GMod, the entire screen is magnified and shifted to the top left of my monitor, leaving only a tiny bottom-right portion of the screen available to me. I can’t access any of the menus or the dev console. This problem doesn’t happen with any other game. How can I fix it?

Try right clicking your Garry’s Mod in your library, go into Properties, and Set Launch Options - In there you can type, “-w ####” and “-h ####” (replacing #'s with the numbers and no quotes of course) until you get a good resolution that fits your monitor so you can go into the options menu.
Should work, it’s been working for me before.