Touche. Yeah, not really a gambit since the pawn is easily regained.
What are the only gambits gms play?

http://www.academicchess.org/images/pdf/chessgames/fischerbust.pdf
Apologies, I was referring to the comment re the Marshall...I've amended my post.
GMs would play the Marshall Gambit if allowed. Nobody, however, allows either Marshall Gambit. They do not play 8.c3 against 7...castles and nobody at the top plays the Triangle Defense and allow the Marshall Gambit by White, which is 4.e4. Nobody plays the Muzio any more either. 4...Bg7 advantage Black instead of 4...g4. Only time Black should play g4 early on is if White plays h4.
Could you elaborate on that as my database seems to disagree, unless I am misunderstanding the position you're describing.
The position ThrillerFan describes is the Bg7 move after white castles in the Muzio gambit. You can read Fischer's paper on his "Bust to the King's gambit".
http://www.academicchess.org/images/pdf/chessgames/fischerbust.pdf