I'm not an ornithologist and have no idea how many species of gulls there are or whether they're all protected, but yes a number of gulls, if not all, are protected in the US by the Migratory Bird Treaty Act of 1918 plus various amendments. There's a list here:

https://www.ecfr.gov/current/title-50/chapter-I/subchapter-B/part-10/subpart-B/section-10.13