Its very simple:
1. get the id of the addon you want the gma of. (its in the url)
2. open up your Garrysmod Folder
3. go 2 directorys up until you see the folders named "common" and "workshop"
4. open the folder named "workshop"
5. open up the Folder with the id "4000"
6. now search for the addon id you copied in the first step and open the folder
7. now open the temp.gma file with the gma.exe located in the garrysmod "bin" folder
8. Profit