If White will just get a pawn back at no cost, why is the
Queens Gambit a
Gambit? Is it for historical reasons or something else?
I think that it's just because it looks like a mirror version of the King's Gambit, nothing more.
The fact that a gambit involves a sacrifice in no way means that the sacrifice is permanent. Very few mainline gambit openings for White leave the gambit player with no means of retaking his material.
While Wikipedia does not necessarily make the distinction between a gambit and a sacrifice, I would contend that there is a difference:
Based on this understanding, there is no reason NOT to consider the Queen's Gambit as a true gambit, unless you wish to state arbitrarily that there must be more than X number of plies between the loss and the subsequent regaining.