Why does no one remember Gildartz?
He is mentioned. In chapter 119, Mystogan mentioned him to be the strongest of Fairy Tail and I quote:


  Mystogan: I have little interest in such things, but I would propose Gildartz.
  Laxus: Eh, he's no use... He ain't coming back.


Presumably, Laxus knew Gildartz was undertaking the 100 year job involving Acnologia and had assumed him to have died, since no one else has ever come back from that mission alive.