What is a GMA file?
A .GMA file is primarily a Garry's Mod Add-on archive. The game Garry's Mod uses these files to store custom content like 3D models, textures, sounds, and scripts. Facepunch Studios created this format to make it easy for players to download and install game modifications through the Steam Workshop.
However, .GMA files can also serve other purposes. Some are Garmin map authentication files used by Garmin BaseCamp to verify digital maps. Others are Graitec Model Archive files, which are compressed as 7z archives, used by Graitec Advance Design.
How to open GMA files?
To open a Garry's Mod Add-on, you usually let the Steam client handle the installation automatically. If you want to extract the files manually, you can use a command-line tool called gmad.exe, which is included in the game's installation folder. There are also community-made GUI tools like GWTool to extract the contents without using the command line.
For Garmin map authentication files, you open them using Garmin BaseCamp. For Graitec Model Archives, you can use standard archive tools like 7-Zip to view the internal structure.
Using viewer.online for GMA files
Because a .GMA file can be a game add-on, a map authentication file, or a 3D model archive, it is sometimes hard to know exactly what your file contains. viewer.online/gma can analyze .GMA files to identify the exact format and creator software, inspect the file structure, extract readable text, and check whether an online preview is available.
Summary
viewer.online/gma is useful for identifying, inspecting, and understanding .GMA files without installing software or dealing with compatibility problems.