I did multiple anylasis of 1. e4 e5 2. f3 Nf6 3.g4 and at this point black is supposedly at an advantage, but after 3. ...Bc5 4. g5 Nh5 5. h4 Bxg1 6.Rxh1 Nc6 7. c3 b5 which is a sequence I have seen frequently, white is actually supposed to be at the advantage, more so after 8. b4 O-O 9. d4 ...
So the 2. ... Nf6 3. g4 route shows some promise.
1.e4 e5 2.f3 is an absolute nonsense. If it works in OTB maybe you must stop playing in the ignorants/ beginners/ "just learned the moves" division.
Probably he plays in super fast time controls, too.
So the 2. ... Nf6 3. g4 route shows some promise.
Unfortunately, this is losing for white cuz of the same famous Damiano sac 3...Nxe4(!!) 
evaluated at about -5 by SF9+. 
1. e4 e5 2. f3 Nf6 3. g4 Nxe4 4. fxe4 Qh4+ 5. Ke2 Qg4+ 6. Nf3 Qxe4+ 7. Kf2 Bg5+ d4 Bxd4 8. Nxd4 Qxh1
1. e4 e5 2. f3 Nf6 3. g4 Nxe4 4. fxe4 Qh4+ 5. Ke2 Qg4+ 6. Nf3 Qxe4+ 7. Kf2 Bg5+ d4 Bxd4 8. Nxd4 Qxd4 9. Qxd4 exd4
In the end black has lost a queen, a bishop, and a knight in exchange for a queen, a knight, three pawns, and control of the center of the board.
5...Qxg4+(??) is a blunder, the correct and very strong move is 5...d5(!!)
Look at what I corrected it to, is it still really a blunder?
Keeps queen and gains a rook.
hey
Hello.
Look at what I corrected it to, is it still really a blunder?
Well, it's not an absolute blunder, since after 5...Qg4+ black has still an advantage (ev=-1). But it's a relative blunder with respect to the significantly stronger move 5...d5 (ev=-5). So, U see, the difference is quantitatively equivalent to 4 pawns.
What is so good about 5. ... d5?
What is so good about 5. ... d5?
Black opens the path to its LSB (light-squared bishop) threatening with Bxg4+ and white has no good defense.
So, it's 2 extra steps to what I posted and improves black's winnings, ok.
So, all things considered, not 3. g4. We may make some real theory yet!
So, all things considered, not 3. g4. We may make some real theory yet!
Yes, vs. 2...Nf6 (Reversed Damiano defense), the best choice is 3. d4 leading to what we called Fried Potato Attack: Two-Knights defense (ev=-0.5). 
[cf. my diagram, post #47]
I have done 3. d4 repeatedly as well. I usually use it to stall, waiting to see where to attack.
However, black can also play 2...Bc5 (Italian variation). What's the plan in this case ?!? The Evans gambit 3. b4 seems too risky. 
That gets done a lot. If you respond with g4 now then black cannot effectively sack the knight.
Current Summary of the King's Head (ev=-0.8) variations: