It surely looks like eighter a kelp or a cape gull as the mirror on p10 and broad trailing egde rules out GBB. But when is comes to kelp (dominicanus) vs cape (vetula) (considered conspecific by some) it gets more difficult. The white mirror is hard to use as a clear indicative, but the dark eye, the olive tigned legs and the appearent flat crown point towards cape gull. Other indications are the very dark back as you mention, but the lower mandible also shows an obvious angle at the gony spot, which points towards cape. Any more pics of this gull?