Can you tell me why in this position Nf6 is a good move and black is winning after this move?

ok let me give it a try. 

Variation A

1. gxf6, gxf6 2. ke4, kc5 3. kf4, kd5. King has to move out of the way, thus black winning a pawn and the game.

Variation B

1. g6, Ng4 2. ke4, ke7 3. kf4, kf7 4. ke4 Nh6 5. kf4 Nxf5. And it shouldn't be to hard for black to win the other pawns and win the game.

tell me if I missed something

Yes. The King can also go to Ke5 at 4th move in variation B. Then what will black do?

 And thanks for your answer.

coz the c4 pawn will be taken by black in move 6!

Sorry I made a mistake here. 4.Ke5 is an illegal move. So Sebas4life has given the right answer.

         Thanks to both for their answers.