King gambit nf6?

After e4 e5 f4 exf4 nf3 why is nf6 for black a good or bad idea?

This is called Schallop defense and it's a good move.  Here is the main line:

followed by 5...d5 or 5...d6.

Is it ok if you explain why its good? Doesn't this just give white even more central space?

White has difficulties to recover f4 pawn, and has no target (maybe Nh5 on the rim, with Be2 as an option for white, on move 5 for example after Nh5), so black is quite fine.

Of course, concrete analysis is needed.

For more information, just purchase this book, one of the rare one which analyze it, at least from black's point of view :

https://www.amazon.com/Playing-1-e4-e5-Classical-Repertoire/dp/1784830143/ref=sr_1_1?ie=UTF8&qid=1490101079&sr=8-1&keywords=ntirlis+chess

IM poucin has just explained it. Well, let's continue the main line, e.g. like that

Nf6 is quite good for Black (after 4.e5 - Nh5 only ) It's true that as a rule it's not easy for White to retake on f4. As White, I like the idea of g2-g3 (with the black pawn on f4) - especially in case Black has already played 0-0. Among other things, though, Black must be ready for aggressive (4.e5 Nh5) 5.Qe2!? - although good for Black on condition of Black's accurate play - but judging from what I faced, especially in blitz games (but not only in blitz) - just a little mistake may be enough for Black to have a disaster (like in the King's Gambit in general, for both sides).

Once in an on-line tournament on the schacharena.de (time control was 1 hour/game) in 2008 or 2009, as Black, I also played the Shallop defense 3. ... Nf6, and my opponent did play 4.Nc3  I had seen one game where B. Spassky played so at one of Chess Olympiads. 4.e5 and 4.Nc3 are quite equal in strength, but the pawn move seems more tricky and more poisonous for Black, just from my impressions. As for Black, after 2.f4 ef 3.Nf3, formerly I played mostly 3. ... g5 and from time to time 3. ... Nf6. Now I am practicing the Fischer defense 3. ... d6

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