I recommend you either Blender or 3dsmax, also you need to install source plugins for theese programs. Also you need to have Crowbar decompiler http://gtm.steamproxy.vip/groups/CrowbarTool/discussions/0/617328415069261839/?ctp=2#c135507548124509682 to compile models.

I'm using GAC, here is a link https://facepunch.com/showthread.php?t=1385203