Yeah this is a stupid question. But how do you add hats to TF2 Characters in Gmod? I’ve tried using the body group tool and all I get is hat-less.

You don’t.
Hats are on attachment points, they’re not part of the actual model. (As in there isn’t a heavy model for every single hat)
Spawn the hats from props and then constrain them. Alternatively use holograms, or lua to place and parent hats.