The Grey-headed Gull is distributed in Ecuador and Peru as well as Brazil and Argentina. Another sub-species is found in sub-Saharan Africa. See the distribution map at Birdlife International.

I include this rather poor photo taken near Uruguaiana, Rio Grande do Sul, Brazil (I was actually trying to photograph the Great Grebe Podiceps major at the top of the image) because Belton says that this gull is rare in Rio Grande do Sul mentioning only 5 records. He also suggests that it may be more common than this given the difficulty of distinguishing it at rest from Brown-hooded Gull L. maculipennis. In flight it should be easier to identify and here you can see the smallish white "windows" on the black primary tips. The Brown-hooded Gull has more extensive white on the primaries with small black tips on all but the outermost primaries.