How do I make a script that if you click a GUI it gives you a hat?

Hellu. My name is moosee on roblox, I've been scripting for a while, but I still can't figure this out.

So, I am making a developer product for my game that gives you different hats, I can't figure out how to make it so when you click that GUI it gives you the hat. I know how to make the hat show up on the characters body, but I don't know the GUI part. Please help!