“Gandalf… yes, that was my name. I was Gandalf.”

[luthien-elvenprincess]

Gandalf was known different names by different races. The elves knew him as Mithrandir, therefore Galadriel wouldn't have called him Galdalf. Treebeard, being so old, probable knew him as Olorin, or at least, Mithrandir since the Elves taught the Ents to speak. So, Treebeard probably didn't refer to Gandalf as GAndalf when he met him as The White.

Gandalf met Aragorn, Gimli and Legalos on March 1, and then met Merry and Pippen on March 3 (see appendix B)...therefore, Aragorn's "Gandalf" exclaimation was indeed the first time he heard it since arriving back.