Found this handy tool on garrysmod.org that lets you place models in GMod and write them to the vmf of the map.
Instructions
1. Place vmf in data folder
2. Rename mapname.vmf to mapname.txt
3. Select the props you want to add to vmf using left click
Red - prop_dynamic
Green - prop_static
None - will not be added to vmf
4. Reload to write the vmf
5. Locate mapname_withmodels.txt in data folder
6. Rename mapname_withmodels.txt to mapname_withmodels.vmf
7. Open with hammer
Link :
http://www.garrysmod.org/downloads/?a=view&id=70844
