In spite I first identified the bird as a Caspian gull (Larus cachinnans), some comments have allowed to repair the mistake. Many thanks to Chris Gibbins, Visa Rauste and Theo Muuse!!
A short abstract of these comments is written below:
Caspian gull at this age stage should show white mirror on P9 and in this bird this is only visible in P10 (still growing in the 6th picture). In addition, the mantle is dark, the legs are short and the gonys angle is strong. In spite all the big gull species show variation, there are too much “bad” things for Caspian which are more important out of the species range.
However, the pink legs, the dark eye, the head and bill structure neither fit with michahellis. In addition, the extent of black in the primaries (up to P3) and the dark rachis in secondary coverts are also strange.
The final conclusion is that maybe this bird cannot be identified at species level so we left this as an unidentified gull.