You need Steam open to do this (simply), this is a non-negotiable. If your Steam is somehow broken, skip to 1.1, locating it manually.

Find your GMod in your library (Search is a godsend!)


Right click it just like so and choose Properties. Simple so far. Now I'd like to steer you in the right direction by instructing you to click on the Installed Files option.

After you have done that, now click on the magical option Browse.

This should open file explorer in your Garry's Mod folder. Now copy the path by clicking in the bar where the path is, and Ctrl + C on it.

If you struggled with Steam, say it's bugged and just won't work for you, you can do this method. Open your File Explorer and paste in one of two directories in the top bar.

C:/Program Files (x86)/Steam/steamapps/common

If you have GMod on an external drive,
EXTERNAL DRIVE LETTER:/SteamLibrary/steamapps/common/

Find your "GarrysMod" folder, double click on it and copy that directory.

Now you need to press Windows and search "environment variables".

Click "Edit environment variables for your account."

Click on "New" under User variables for WINDOWS USERNAME.

Call it something like GMOD and paste in that directory in the variable value box.

Click on OK and you are set.

Now to test it out and see what it really does, press Win + R and type in whatever you named it inside of 2 percentage signs (e.g: %GMOD%) and then press enter.

Your GMOD folder will open!

Environment variables are basically custom shortcuts that can be called from anywhere. Even Command Prompt leads to them when you type in cd %GMOD% for example.

These are really, really useful for modders.