These pics were taken on December 20, 2006 in the Coastal Plain near Houston and Galveston, Texas - note that is was the size of a small Herring gull:
The bill shape is too thick and hooked for LBBG - but with perhaps too much of a gonydal bulge/droop for GBBG; the head shape looks more like GBBG than LBBG; the wing-tip:tail-tip:tertial-tip ratios are wrong for LBBG; the bit between the head adn the wings/tail look fine for LBBG; the darkish underwing coverts have pale bands - wrong for LBBG; the tail looks to be mostly black. I wonder if this would stand out among first-cycle Kelp Gulls in one of the nothernmost breeding areas in Brazil or Peru (where the nesting months may be closer to the range of dates known for Northern Hemisphere gulls)- ?:
