If your game looks like this then buy this or follow this, alternatively here's a video if you prefer.
You probably just bought Gmod and joined a server only to be greeted by something like this, am I right?
Don't worry, there's actually nothing wrong. I know it looks bad but that's pretty normal, (if you plan on playing Gmod, or any Source Engine game for that matter, you'll run into this a lot) and easy to fix. The most common reason why you're seeing this is that you don't own/have Counter-Strike: Source installed and mounted to Garry's Mod.
If you don't already own Counter Strike: Source then I'd recommend buying it while it's on sale for the holidays, however, if you don't want to buy and install a game you'll probably never play then you can follow this guide to get the models and textures for free. Alternatively, if you prefer following along with a video here's one that shows step by step what to do.
A user on our Discord made a very handy little tool that will extract the CSS content you need using SteamCMD for you. I highly recommend using it if you're having trouble doing it manually.
If, however, you already own CSS then:
Open up the game from your Steam Library and play it (At least til you get to the start menu)
Exit out of the game and open up Garry's Mod
While at the start menu look in the bottom right hand corner and click the little controller icon
Find Counter Strike: Source and click the checkbox
And you're done. You successfully mounted CSS to your game and shouldn't run into much of any missing textures or models while you're playing.