Hello Killian,
welcome to birdforum!
First one is a 1 winter= 1 calendar year= 1 cycle bird (=that means it was born in 2019). I havent experience with that complex problem in the Middle East, but it looks like one of those eastern type Lesser black-backed Gull forms or a Caspian Gull to me. Strange for a Caspian Gull seems to me the extensive black tail ....
But I better stop, and hope for the experts to jump in. Thankfully there are some of the foremost gullers active around here and they will (hopefully) help us.
Second bird is a 1 cy Common Gull. Please note:
- just imaginable rounded head with tiny bill (against one of the larger Gulls)
- broad uniform tailband without any pattern/smearing/spotting on the base (not good for a large Gull).
This bird looks different to the ones you are used to see in Belgium. But the suspected ssp
heinei in Abu Dabi looks like your bird with a white underwing and a contrasting dark trailing edge.