Hard picture, at least for me it is, but:
inner primaries seemsto show the venetian-blind effect in such a good shape, that this would be unusual for a YLG
underwing seems to be very palw , even whitish. Yes, this can be explained by slightly overxposure and strong light washing out any pattern there
narrow, neat and clear cut tail band
seems to be very white bodied and white headed (difficult to judge with this pictures). Are there more pictures showing head-sides, where it is possible to judge position of the eye, bill and any streaking to head-sides and crown?
Yes, greater coverts should be darker in an ID-book 2 cy (or a 3cy?) Caspian Gull, but this the best fit to me.
I hope for others to jump in and confirm or correct me