From Garry's Mod
Mounts a GMA addon from the disk. Any error models currently loaded that the mounted addon provides will be reloaded.
Location of the GMA file to mount, relative to the garrysmod directory
If successful, a table of files that have been mounted
Downloads the Playable Piano addon and mounts the content.
steamworks.DownloadUGC( 104548572, function( name ) game.MountGMA( name ) end)