If you using linux on X11 you can use scaling for monitor. But gmod can't understand it.
You can only use the maximum resolution your monitor supports.
But if you want to fit more information and still play gmod normally here's what to do:

First open the game folder and change in hl2.sh this line:
export SDL_VIDEO_X11_XRANDR=1
to this
export SDL_VIDEO_X11_XRANDR=0
save and close.

Then add to launch options these launch parameters:
-h <resized height> -w <resized width>

Done. You can play normally with your resized screen.
Thanks for reading and good luck.