Well, VMF is a Valve Mapping file. This, you open with hammer, to edit.
Simply navigate to your garrysmod/garrysmod/maps
Navigate to your Counter-Strike:Source/cstrike/maps
Copy the desired map, so map.BSP (Actual map file)
Paste it in your garrysmod/garrysmod/maps folder.
yup like all the others said and also yeah like valiant said just take the jail_forest_v3.bsp put it in the Gmod>Gmod>Maps folder and its gonna be in others