Adult bird in molt with still growing outer primaries and missing outer secondaries. The dark markings on the bill relate to being non-breeding rather than a younger bird.
The bulky size, large bill, dark back and pink feet make it unique. When seen like this only Slaty-backed Gull (Larus schistisagus) can look similar, but is first of all not present in the area, and will normally appear smaller, more slender and with a more intense feet coloration.
As always the back color differs with the light and viewing angle, here looking considerably paler on the bird when flying away.
Adult scavenging a dead bread in a black plastic bag.
The deep orange eye-ring and gape is probably a result of hormonal preparation for breeding since other adult birds seen on gull-research.org show yellowish eye-ring.