It seems like the vehicle choices in Gmod are bit weak. I’ve seen a couple people port models from GTA4 and they seem pretty decent. After recently checking out car mods for GTA I realized (with permission obviously) someone could port other peoples custom made content to Garrysmod.
If your’re interested here’s a superb website: http://www.gtainside.com/en/news.php
Direct Link to a list of models: http://www.gtainside.com/en/download.php?do=cat&id=372
If you do port anyone’s models ask permission first, I can’t stress that enough