The save files that are generated when you click "Save Game" in the "Saves" tab?
I attached an addon to my post with a function that can load these files on a dedicated server.
1. Add the addon to your server by extracting the archive in your server’s addons/
2. Upload saves (with .gms file extension) to the data/
directory of your dedicated server. (You can find save files in the saves/
directory of your local Garry's Mod installation.)
3. (Re)start your server.
4. Run the following command from your server console: lua_run LoadSaveFile("awesome_save.gms")
Technical information for developers
Save files are binary files with the following data structure:
1. File signature "GMS3" (0x47 0x4D 0x53 0x33
2. Map name as a null-terminated string.
3. Duplication info in JSON, compressed with LZMA.
Entities can be restored by passing the duplication info to duplicator.Paste
Small save file for testing purposes: https://timmy.ws/demos/gms-parser/sample.gms
2016-10-15 Update: Added information about the Garry's Mod save file data structure.
2017-01-07 Update: Attached a refactored and optimized version of the addon.