Exchanged Slav defense. Gambit the c6 pawn. Why not? [Update:] It's called "Tan gambit" after an Alekhine game. Analyzed below. White should dicline the pawn, since. It can be played as a drawing line for black (no queens).


d4 d5 2. c4 c6 3. cxd5 Nf6!? 


This seem to me a good gambit line. I think it's a good way of playing but no one does according to www.365chess.com Some people find exchange variation boring. What is the reason that no one is playing it? 

Take a look at the position after 1. d4 d5 2. c4 c6 3. cxd5 Nf6 4. dxc6 Nxc6 5. Nf3 Bg4 6. e3 e5. Black is very active.

Of course white can avoid it by 3.Nf3. Meaning that slav players still have to know the exchange variation. A gambit does not need to be 100% correct. But a gambit no one is playing? 

(I found it via 1.d4 d5 2. c4 Nf6. With no one I don't mean 0).

[FEN ""]

1. d4 d5 2. c4 c6 3. cxd5 Nf6!? (3. cxd5 cxd5) (3. cxd5 Nf6 4. dxc6 Nxc6 5. Nf3 Bg4 6. e3 e5)


[Update]: I used lichess (poor) analysis feature to analyze some games. [Between strong players].


Nf3 Bg4. [What I would play].


7. Be2 [What I would be most afraid of]. ½-½, Reinhold (2139) vs. Stevermueer (2150)

7. Bb5 [Messy internet game] 0-1, Drozdovskij (2509) vs. Dobrov (2515)


Nf3 e5. [Queens are exchanged].


6. dxe5 1-0, Sandipan (2637) vs. Elorta (2363)

6. Nxe5 ½-½, Vorobiov (2577) vs. Zvjaginsev (2646)


Nf3 Bf5.


5. Nf3 Bf5 1-0, Alekhine vs Tan
In the chessok opening tree there are 28 games with this gambit. Going by the computer evaluation it seems to be playable, but the score is atrocious (>80% for white).

So here you have a gambit that is pretty unknown, scores horribly and can easily be ignored if white is not in a mood for adventures. Also it doesn't seem to lead to dangerous complications as far as I can see. There is no automatic attack on the white king, in the worst case white just returns the pawn. 

All in all not very surprising that it is rarely played.