I think he's trying to import custom CSS weapon models, probably gotten from here, and use them in GMOD, probably using the default CSS weapons that Garry added in.
The process is nearly identical to that of installing skins into CSS, just into the Gmod directory. The same folders (Materials, Models, Sounds, ect.) and files all are routed to the right places automatically (because they'll follow the same, identical directory that leads to these new files), and you'll automatically have the new weapon models and sounds.
I have to warn you, however, that some of the custom-weapons you may be trying to import have different origins than the default CSS ones do: with that in mind, adding custom models with custom origins is REALLY going to mess up your iron sights.
My personal recommendations: don't put in complete custom weapons, but maybe some High Definition Reskins: that way you've got less-ugly guns, and your iron sights aren't so messed up.
Hexing isn't important now, I'm assuming, since you don't seem to be creating new SWEPs that will use custom models. It's always a good thing to know, however...