Peregrine Falcons are probably the major predator in the US, though large gulls are on the upper end of the size range of what they could take. Farther north, the larger Gyrfalcon could be a significant predator. Some of the other larger hawks, or even an eagle, might take one on the odd occasion. Coyotes or foxes would probably grab one if they had the chance, though again a large gull would be a bit of a handful for a fox. In urban areas, the larger gulls probably don’t really have many serious predators, but it doesn’t hurt to hang out in a safe place anyway just in case.
To every fry (tern, tern, tern)
There is a seasoning (tern, tern, tern)
And a time for every purpose, under the Golden Arches
triple swoon
And a swoon for Telemark too – accompanied by a heart-rending groan.
There once was a scientist who had a theory that a certain species of porpoise that lived off the coast of Africa never died of natural causes - they were essentially immortal unless they were killed. In order to test this theory, he photographed porpoise schools by attaching an automatic camera to gulls, sending the bird out over the ocean, and then retrieving it when it returned to the beach.
One day he head just recaptured one of these camera-laden gulls on a narrow spit of land that jutted out into the ocean. When he turned around, to his horror he saw a lion reclining at the base of the spit and blocking his path. But upon looking more closely he saw that the lion seemed to be very placid, and was not making any threatening moves.
Gathering his courage, he clutched the gull to his chest and ran down the spit, leaping over the lion in a single bound.
He was immediately arrested. Why?
He was arrested for crossing a sedate lion with a gull for immortal porpoises.
Oh good lord. All I can say is – Colibri – how you doin’? 
Cause if they hung out in parking bays they’d be bay-gulls.
Sorry, couldn’t resist. I was reading this while listening to the soundtrack of “Jonathon Livingston Seagull.” You know, something or-kestrel.
Sorry again. I know I’ve been bad but you don’t have to give me the bird.
Jeez…whatta loon.
Colibri got to the big ones - I just thought I’d mention that I’ve also seen ravens attack sick and injured gulls.