There are several ways.
One of the cruelest which would not work in this case is to wrap a piece of ‘food’ string and get the bird to swallow it. Then you have a seagull on a piece of string like a kite.
Because this bird is so confident, I would simply feed it small amounts of food getting it to move closer and closer to me…. Eventually I would have to be quick enough to grab or net it. Normally only have 1 chance with wild birds, two if you are lucky. A throw net would also work well. Easier with two because one can keep it distracted while the other nets it.
Never caught a sea gull by the above method but have gotten Mallards and Pigeons before.
I also gotten Gulls and oyster catchers tangled in my line before…. Found a half drowned Oyster catcher one day because I was up in the camp ground packing my stuff away….. Because I had no scissors with me it was quite a strangle to free it… Gave it a 50/50 chance of survival.