I'd have to say client side model glitch, like it loaded the wrong model at first then eventually got the right one.
Even so if he was hacking who doesn't want a gun penis ? Haha but in all seriousness I think this happened to me once or twice pretty sure it's a bug
Well, it's been a bit since I've been on the regular servers and looked through the weapons. I had the model downloaded for a while, but removed it. Oh well, I will have to try it out tonight lol.
It's a bug that has to do with the scripting and the position of the gun. Try verifying the game cache of your gmod files and see if that helps. If not, you may want to uninstall gmod, delete the folder so all the content is gone, and reinstall it to get a fresh start.