After entirely too many posts (don't think it'll stop me in the future) on Gull-nasty, the dark-mantled Chandeleur type gull (Kelp x Herring), I have a different dark-mantled hybrid for your viewing pleasure.
It made a couple close passes at our bread. I kept shooting to try to get the orbital color...
So, to summarize the intermediate characters on this bird, body size and bill larger and heavier than HERG arguing strongly for GBBG, mantle intermediate between HERG and a black-back, orbital intermediate, the feet are pink pretty close to a typical HERG and not like the much paler pink feet of a GBBG. The intensely pink feet, large size, and minimal head markings are pretty strong arguments against LBBG as the black-backed parent.
While it's always hard to compare shades of gray in a photo, I think that these pics are pretty representative of the relative shades, here's a Lesser Black-back from the same flock at virtually the same time:
Lesser seems to moult later than do the other gulls, this one is a bit asymmetric with an old P10 on one side and a fresh P10 still growing in on the other.