The GMA file format is a package format. It contains the Garrys Mod game content (materials and models). You get this file when you subscribe an addon from Garry’s Mod.

You'll find this files by default in:
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons

First – you don’t need a thirdparty tool to unpack a GMA file. The extraction tool is a part of the Garry’s Mod game installation: gmad.exe

Create a shortcut to the tool gmad.exe, found (by default) in
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\bin

Step 1:
Create a new folder

Step 2:
Find the addon you want to extract and copy the gma-file in your folder

Step 3:
Drag the gma file on your gmad.exe shortcut.

A command prompt run and the selected addon file should be extracted in a new folder, with the name of the extracted workshop addon.

Now you can decompile the model-files (found in the folder ‚models‘) and import them in a 3D-tool of your choice.


Notify:
Get the permission of the addon-creator for using addon-parts!