It's Yorki. Everyone else in Brook's flashback were blatantly shown to have died in the attack that killed Brook. Everyone except Yorki, who was last shown sick, but alive, sailing away in the hope of surviving. Why did Oda leave his fate, out of all the Rumbar Pirates, ambiguous? Cause he's actually alive and waiting with Laboon as a final surprise and reward for Brook.
That guys hair, which is basically the only thing pointing at it being Gaban, in addition to being blonde, isn't anything like Gaban's hair. Gaban's hair is absolutely wild, it looks like a giant hedgehog has fastened itself to the back of his head
https://vignette.wikia.nocookie.net/onepiece/images/4/41/Gaban's_Axes.png/revision/latest?cb=20191225013135
https://vignette.wikia.nocookie.net...ars_Ago.png/revision/latest?cb=20191225020552
This guys hair is long, but quite tame.