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

[Morsul the Dark]

Ah but the book is flawed as well we can argue of whether or not Merry and Pippin see Gandalf prior to them arriving at Isengard (though there is already a quote above stating they did) But he also was at loth lorien where he got his white cloak and such certainl Galadriel would tell him his name.