.GMA File Extension

Addon file used by Garry's Mod, a sandbox game that revolves around building random objects; contains data that modifies gameplay, such as character appearance, game modes, weapon skin, environment layout, maps and more.

You can create your own GMA files or download other users' add ons. You can download them through Garry's Workshop (gaddons.org) or through the Steam Workshop. Once downloaded through the Steam Workshop, the file is located in the garrysmodgarrysmodaddons directory of your game. If you don't download the GMA file through the Steam Workshop, you'll need to manually place the file in the aforementioned directory.