What is .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 \garrysmod\garrysmod\addons\ 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.
You can see a list of software programs here on easyfileopener.org that can open this file type so that you don’t need to search for it on the web. You can then open your file with this extension using a software listed below.