First of all I recommend to launch the game as "Windowed" in display mode, and put the game as the smallest resolution possible, as seen on the picture:

I'll use the screen command so you might want to install it:

On Debian/Ubuntu (and other derivatives) :
On Arch Linux (and other derivatives) :

Now, you can close the game and go to your Desktop or other folder of your choice (doesn't matter) and create a bash file

Filename example:

And then, insert this in the file:


You will need to change the path to your GarrysMod folder (where the executable is) (line 5)
To do so:

• Right click on GMod in your steam Library
  
• Properties
  
• Local files
  
• Browse Local files

And you will be inside the folder, you can copy the path and past it in the script.

Also, you might want to change the number of launch it will make.
To do so, just edit the second line and change '1000' to whatever number you want.

On SSD, the game might launch faster so you can adapt the speed at which the script close the game with the line "sleep 9" to whatever works for you.

Once everything is setup correctly, you can make the script executable and launch it, to do so, you can use the terminal:

Or you can do it via your GUI by doing a Right click on the file, then Properties and in the permissions area, click the "Allow executing file as a program" (this process can vary from a Desktop Environement to another)

Now, profit