They are capable of flying considerable distances. Franklin's gull migrates from central Canada to the lower portions of South America routinely. Still, most seagulls prefer to stay close to land for nesting and easy access to food, as they are scavengers often found around landfills and waste dumps, picnic areas, and any place people drop food.
The ability to scavenge food from land or dive into water for it is only the first reason gulls can travel theoretically unlimited distance over time, and why they are found on every continent, including Antarctica. Gulls have the ability to drink both fresh and salt water, so dehydration is never an issue for them. They are also very energy efficient, sometimes being able to hang in winds without moving at all.