Symmetrical Boi-Lopez: Delayed King's Gambit

How good is the following fancy opening?

It appeared in Greco-NN (1620) games/compositions:

http://www.chessgames.com/perl/chessgame?gid=1272154

http://www.chessgames.com/perl/chessgame?gid=1272155

http://www.chessgames.com/perl/chessgame?gid=1272156

http://www.chessgames.com/perl/chessgame?gid=1272157

Stockfish evaluates it around the strict 0 and suggests 4...Nc6 or 4...d6. On the other hand, 4...exf4?! and 4...Bg1?! give an advantage to white.

Let's consider the Lopez variation: Delayed KGA:

The correct move here is 5. Nf3.

Newbie guess: White wouldn't have a good castle on the right?

Yeah, that's quite possible as it happens in many standard KG lines.

We consider now the following position (King's Pawn game: Symmetrical Boi-Lopez variation: Delayed KGA: main line):

NN played 5...g5 but, contrary to the usual KG, it's a mistake here. Let's analyze it.

d4 is coming so Nc6 I guess.

Good guess. The countergambit 5...d5, opening the way to the black's LSB, is also quite reasonable.

And if after Nc6 white tries to prepare d4 with c3? A good time for the d5 thrust?

Or Nf6...

• Yes, 5...Nc6 6. c3 or 6. Nc3 threatening with the attack 7. Nd5.
• After 6. c3, 6...d5 isn't good any more. Local Stockfish suggests 6...Na5 or 6...Bb6.

hm looks good for white to me.

Yes cuz 4...exf4?! was already a black's inaccuracy (contrary to the classical KG).

Not, of course. The best moves are 3...Nc6, 3...Nf6 and 3...d6.