yeyeye ik many ppl asked on SFM and this Gmod. So.. I can help you how to add shadows.

Here's an example this Screenshot:
http://steamcommunity.com/sharedfiles/filedetails/?id=898796083

First, Decompile as mdl file but idk which tool your using it. Next, open qc file here's an example:
$ModelName "DrJintei/jintei.mdl"




Ok, add "$MostlyOpaque" this means shadows will be enable by itself. Here's how you add:
$ModelName "DrJintie/jintei.mdl"

$MostlyOpaque


Done, save it and recompile that, and all your cars works shadows.

http://steamcommunity.com/sharedfiles/filedetails/?id=444906819
http://steamcommunity.com/sharedfiles/filedetails/?id=406114553
http://steamcommunity.com/sharedfiles/filedetails/?id=356096311