Why is the Queen's Gambit not a "real" gambit?

Would someone mind explaining why the Queen's Gambit is not considered a real gambit by almost everyone here?

Because white can regain the gambit pawn very quickly, black cannot defend it.

Also, typical postitions are usually more closed in the QG, which is the opposite of what one generally tries to achieve with gambits.

Just because it leans toward closed positions is no reason to not call it a gambit.

Well, since white can regain the gambit pawn quickly, I guess it isn't as much of a gambit as the others.  Fine.

this is why

Bronstein used to play the Queen's Gambit in gambit style.  It is fairly rare for black to accept the gambit with the idea of holding onto the pawn.