How to get a custom model from a different game into garry's mod?

Well I spend my time going through gamebanana looking at stuff to hex, skins to download, you know all of that kind of stuff. However I have seen some interesting models from other games such as Battfefield 2 and Grand Theft Auto. How would I get these props to work in garrysmod? I have the time to do it if much time is needed.

By ripping|porting them whatever and converting to source.