Use the official Garry's Mod tool called gmad.exe, found in your game's bin folder, to extract the .gma file into a normal folder structure. Drag the .gma file directly onto gmad.exe or use Command Prompt to run the extraction command.
Where to find the extract gma file tool
Navigate to your Garry's Mod installation directory, typically located at C:\Program Files (x86)\Steam\steamapps\common\GarrysMod if you installed through Steam. Inside that folder, open the bin subfolder. You will find gmad.exe alongside other engine utilities. If you cannot locate it, verify your game files through Steam to restore any missing binaries. This executable is the only official tool for handling .gma files, and it ships with every copy of Garry's Mod, so there is no need to download gmod extras or third-party software.
Extracting with drag and drop
The simplest method is to locate your .gma file in File Explorer, then click and drag it directly onto the gmad.exe icon. Release the mouse button, and a command window will flash briefly. The extracted folder will appear, typically in the same directory as the original .gma file or as specified by the tool's behavior. Inside, you will find a subfolder named after the addon's internal identifier, containing the complete file structure. This approach works for the vast majority of addons, including those you might manually download from Steam workshop using third-party tools, which is especially useful if you need to download Steam workshop mods without Steam on a restricted machine. If you need to extract multiple files, you can select several .gma files and drag them all onto gmad.exe at once; each will be extracted individually.
When extraction fails or the file is corrupted
If dragging the .gma file onto gmad.exe produces no result or an error message, the file may be missing a critical component. The most common error is "missing addon.json", which means the archive lacks the metadata file that defines the addon's structure. This often happens when someone tries to share a workshop download cache file rather than a properly packaged addon. Such cache files are not real .gma archives and cannot be extracted by any tool. Another frequent issue is that the .gma file was created for a different game or engine version and may refuse to open. If you have downloaded the file from an unofficial source, corruption during transfer is also possible. In these cases, verify the file size against the original listing and try re-downloading. If you need to obtain the addon again, remember that you can manually download from Steam workshop using browser extensions or dedicated sites, but ensure you get the actual .gma format, not a cache file. If gmad.exe still fails, the file is likely damaged or mislabeled, and no recovery method exists. Unlike other guides, this page explains the specific “missing addon.json” error and why workshop cache files are not real .gma archives, a distinction that prevents wasted extraction attempts.
