The ultimate gnomish hat is the "Cat In the Hat" hat. Any person actually sporting one of these at a public event is a fucking gnome. Anybody even owning one of these is a gnome, and you should be immediately on guard about this person you only thought you knew.
The runner-up to this hat is the beer helmet, which weds the gnomish tendency toward pointless whimsy with irritating contraptions (to be dealt with later).
Snowboarders are particularly susceptible to gnomish headgear, for reasons as yet unexplored. Maybe it's part of the devil-may-care culture endemic to snowboarding? I don't know.
In the case of this picture below, the girl is most definitely the gnome, here -- can't you see her smug smile, there? "Look what I did?!" She even looks like a gnome (the eyes are a dead giveaway, although I blocked them out, here). No doubt she crafted those wacky, wacky hats in a fit of contrived wackiness that plagues gnomes everywhere...