Also, has this ever been played before in master games?
Tarrasch defense gambit

No, it isn't. The reason is rather simple: 5.e4 gives white an advantage. As usual, accepting the pawn immediately gives Black compensation. Enough, or not enough- who cares?
It hasn't been played in master games according to game explorer. I don't know how good it is for Black. It looks fine but I'm really not the person you should be asking about openings.

How does e4 give white an advantage? If black plays correctly, then he can still be ok, if not better.

well im no tarrasch defence expert but im not sure you have enough for the pawn. but there is another gambit here you might be interested in which is called the Hennig-Schara gambit which is very complicated
Personally i wouldnt bother with any of them for something else than a supriseweapon because the way i understand white can moveorder around those gambits with 2.Nf3 pretty much forcing you into normal tarraschlines. If you are forced to learn the normal tarrasch anyway why not right away
I was looking at the Tarrasch defense, and I noticed that a gambit could be played. I analyzed the position a little and found one way that it could go where black has a clear development advantage.
Is this good for black? If white takes the c pawn, then black trades queens and takes the c pawn with the bishop.