Generally, the Queen's gambit it's not considered a true gambit, because white comes out ahead with an accepted queen's gambit. White is offering a wing pawn in exchange for a center pawn.
Theoretical disadvantage of an accepted Queen's Gambit:
- P-Q4 .. P-Q4d4 d5
- P-QB4 .. PxPc4 dxc4
- Q-R4+ .. P-QB3Qa4+ c6 (or Q-Q2Qd7, or B-Q2Bd7, or N-Q2Nd7)
- QxPQxc4
Now white has a commanding control of the center of the field, and is not down any pieces. (though the queen is out and exposed very early)
At least that's how it was explained to me 20+ years ago.
Not that this is the way it's usually played, but white players consider that other things they can do with an accepted queen's gambit are better than this.