Gulls tend to go wherever they find food- and as the first answer says, landfill sites are ideal! So are park lakes, ploughed fields, macdonalds car parks- loads of discarded food there! The two most common gulls- the black-headed and the herring gull- are the ones you see most often inland- both are more common than the Common Gull! They will both breed quite happily by inland marsh/lakes so no need for them to ever see the sea. Outside the breeding season they will go wherever the hunt for food takes them.