Positionally White has just blundered a little by playing
f6 takes advantage White neglecting to develop a piece and begins to work on attacking
d4 from the king side ( obviously there is pressure on
d4 from the queenside as well). Black is listed as having a slight
-0.28 advantage (23 moves).
A player who is not experienced defending may lose to a king side assault after moving
f6. However, to build the attack it requires White to sac a pawn, and that is all an experienced player needs sometimes to win the game. This would not be a gambit. Black is very strong at this point and
a3 should seriously be reconsidered.
Play defensively. Although the position favors Black, it is still very slight, and an equal slip by the opponent will even the game out. No material has been lost yet. It is very important not to give in to Black's traps. Of which, there are many here, and all include dropping a pawn. Avoid taking first, unless it is absolutely forced. This game is going to continue on with White being forced over and over to make the correct move in order to properly develop and retain equal material.
There are, however, a few traps you may entice your opponent into
Offer a "free" pawn for a positional advantage.
This is a standard continuation of the position in question. Black will realize that by fiancettoing king side they can possibly win the pawn on
1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. a3 f6 7. Bd3 fxe5 8. dxe5 c4 9. Bc2 g6 10. Nbd2 Bg7
At this point, allowing Black to take the pawn gives White an undeniable advantage. Here is a continuation if Black takes.
1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. a3 f6 7. Bd3 fxe5 8. dxe5 c4 9. Bc2 g6 10. O-O Bg7 11. Nbd2 Nxe5 12. Nxe5 Bxe5 13. Nf3 Bf6 14. Re1 Ne7 15. Ba4+ Kf8 16. Ne5 Kg8 17. Qf3 Nf5 18. Nd7 Bxd7 19. Bxd7 Kg7 20. a4 Rad8 21. a5 Qd6 22. Bxe6 Rhe8 23. Bf4 Qc6 24. Bxf5 gxf5
Otherwise, you may be looking at a long grueling, semi closed position:
1. e4 e6 2. d4 d5 3. e5 c5 4. c3 Nc6 5. Nf3 Qb6 6. a3 f6 7. Bd3 fxe5 8. dxe5 c4 9. Bc2 g6 10. Nbd2 Bg7 11. O-O Nge7 12. Qe2 Qc7 13. Re1 O-O 14. b3 b5 15. Bb2 Bd7 16. a4 a6 17. bxc4 bxc4 18. Ba3
What it all boils down to is, don't be overly aggressive, play defensively, and try not initiate a material trade (especially with pawns) when in this position. In my opinion, playing
a3 is what makes
f6 seem so disruptive, when the real result was a loss in tempo.