Hello this is my first post ever in this forum!.
I chose to reply on this subject because i made some hard work on 5. g4 and i wanted to share the results on what i think it may be considered now a closed chapter.
Now that 5.g4 has lost the surprise effect and the smoke has clarified a bit i would say that this line is still playable for those who love murky positions whithout much theory.
Black's best seems to be the Aizmaparashvili's recipe but ,in my games, black played 6. ... exd4 most of the time. In fact they never played 6. ... Ngf6 against me.
For those of you who still prefer the unclear positions you get in this line, rather than going for the main line,(where i think white has better chances to get an edge). I post here one game i played against an strong opponent adding some of the analysis i have made about this opening (I wanted to put everything but it was four pages long

). The game focuses on 6. ... exd4 because it's the move i faced all of the time. It looks as if white is winning everywhere, but remember that 6. ... Ngf6 is a good reply leading to unclear play.
It has been an epic struggle to show all lines. First of all i want to apologize to color blind people

. And in second place as i am getting out of time i haven't been able to translate the moves from Spanish so... R= King,D=Queen, T= Rook, A= Bishop, C= Knight.
Felipe vera pons - Joan canal oliveras [C41]
Spanish regional team competition.
[Felipe Vera Pons]
1.e4 e5 2.Cf3 d6 3.d4 Cf6 4.Cc3 Cbd7 5.g4 Cxg4 This is the best move in my opinion. Others are...
5...g6 6.g5 Ch5 7.Ae3 Ag7 8.dxe5!?N Felipe vera pons. I think this move clarifies matters a bit. White should be slightly better. 8...Cxe5 (8...dxe5 9.Dd2 f5 (9...00 10.000 c6 11.Dd6 Te8 12.Ah3) 10.exf5 gxf5 11.000) 9.Cxe5 Axe5 10.Dd2 00 11.f4 Ag7 12.Ae2 Te8 13.Af3 c6 14.000 Da5 15.The1 Axc3 16.Dxc3 Dxa2 17.Txd6;
5...h6? 6.g5! Now black is fighting for survival. 6...Cg8 (6...hxg5 7.Cxg5 Ae7 8.Ac4 00 9.Tg1 c6 10.d5 b5 11.Ab3 b4 12.dxc6 bxc3 13.cxd7 cxb2 14.dxc8D bxc1D 15.Dh3 Da5+ 16.Rf1 Da6+ 17.Rg2 Dxg5+ 18.Rh1 Df4 19.Txg7+ Rxg7 20.Dg1+ Cg4 21.Dhxg4+ Dxg4 22.Dxg4+ Rh6 23.Tg1 10 Filipovic,B-Jurkovic,A/Zadar 2003/[,.] (23)A very nice finish!.) 7.Ac4 c6?! 8.dxe5! b5? (8...dxe5? 9.Axf7+; 8...Cxe5? 9.Cxe5 dxe5 10.Axf7+) 9.Ab3?? (9.Axf7+!! A very obvious "novelty" that wins by force. 9...Rxf7 10.e6+ Rxe6 (10...Re8 11.exd7+ Dxd7) 11.Cd4+ Rf7 12.Cxc6 Db6 13.Dd5+ Re8 14.g6 Ce5 15.Cxe5) 9...Cc5 10.exd6 Dxd6 11.Dxd6 Axd6 12.Ae3 Cxb3 13.axb3 hxg5 14.Cxg5 f6 15.Cf3 Th3! Black managed to draw this position.Sadly i have lost the game data and i cannot refer you to the game.Surely a search position in Chessbase will do?.
5...exd4 6.Cxd4 Cc5 7.g5 Cfd7 8.Ae3 Cb6 9.Df3 (Another idea would be 9.f4 De7 10.Df3 Followed by long castle.) 9...g6 10.h4 Ag7 11.000 De7 12.Ag2 Ad7 13.The1 000 14.Rb1 c6 15.Dg3 The8 16.f4 ½½ Gongora Reyes,M-Moldovan,D/Marin 2004 (42)
6.Tg1 exd4
The top guns prefer 6...Cgf6 7.Ac4 exd4 (7...h6 8.Ae3 c6 9.Dd3 The main line is 9...b5 (9...Dc7?! Shirov stated that white had the advantage in his analysis for Chess informant.½½ Shirov,A-Azmaiparashvili,Z/Plovdiv 2003 (52). (14)) 10.Ab3 (10.Axf7+? Rxf7 11.dxe5 dxe5 12.Cxe5+ Rg8 ) 10...Dc7 11.000 With compensation(Shirov.) 8.Dxd4 with compensation (Shirov). One example went 8...Cb6 9.e5 De7 10.Ae3 dxe5 11.Cxe5 Cxc4 12.Cxc4 Ae6 13.000 c6 14.Ag5 Td8 15.Df4 Axc4 16.Dxc4 Txd1+ 17.Txd1 De5 18.Axf6 gxf6 19.Dd3 Ah6+ 20.Rb1 00 With compensation.10 Solovjov,S-Kirillova,V/St Petersburg 2004 .
7.Cxd4 Cde5
Better than it looks but not very good is 7...Cxh2?! 8.Ae2 Ce5 9.Af4 c5 10.Cb3 Dh4 (10...Chg4!? 11.Axg4 Axg4 12.Txg4 Cxg4 13.Dxg4 white is better) 11.Ag3 Dh6 12.Th1 Ceg4 13.Cb5 Rd8 14.Cxd6 Axd6 15.Axd6 Cxf2 16.Rxf2 Cg4+ 17.Axg4 Axg4 18.Dxg4 Df6+ 19.Rg2.
8.h3 Cf6 9.f4 Cc6
[9...Cg6! Looks better to me, shielding the g7 pawn and hoping to win a tempo with Be7 instead of g6 and Bg7. I think this is the way to go for black if he takes on d4. 10.Ae3 De7?! Black should not waste any time.
10...g6 11.Df3 Gives white the kind of position he wants.Is this sufficient for an advantatge?. For me this line is all for the surprise effect plus the practical chances you get. I would say white gets the type of "advantage" he is looking for.
10...Ad7 This is a natural move but white's compensation here looks very consistent. 11.Df3 g6 12.000 Cxd4 13.Axd4 Ac6 (13...Ag7?! 14.e5! dxe5 15.fxe5 Ch5 16.Ac4 00 17.e6 Axd4 (17...fxe6 18.Dxh5 Axd4 (18...gxh5 19.Txg7+ Rh8 20.Te7+ Rg8 21.Tg1#) 19.Txd4) 18.Txd4 Ac6 19.exf7+ Rg7 20.De3 Df6 21.Tf1) 14.Ab5! Ag7?? (14...Axb5 15.Cxb5 Ag7 16.e5) 15.e5 Cd7 16.Axc6 bxc6 17.e6! 10 Pitl,G-Kunin,V/Bad Woerishofen 2004/[,.] (25)
11.Df3 g6?
11...Cxd4 12.Axd4 g6? (12...Ad7) 13.000 Ag7 14.Cd5 Looks winning for white.
11...Ad7 12.Ab5!? Was my idea at that time.It doesn't look so bad e.g 12...Cxd4 13.Axd7+ Dxd7 (13...Rxd7!? This is a suggestion by deepfritz 8.I hate you deepfritz!. 14.Axd4 Cxe4 15.000 Cxc3 16.Axc3 Resulting in the type of murky positions computers like and humans don't.Still it seems to me that white is better (Deep Fritz agrees after some thought).) 14.Axd4 I think white still has good compensation due to his superior development.
12.Cxc6 bxc6 13.e5 Cd7??
13...Cd5 Is the only move,but white is better, of course.
14.Dxc6 Tb8 15.Cd5 Dh4+ 16.Af2 And it's all over. 16...Dh6 [16...Dh5?? 17.Cf6+!] 17.Cxc7+ Rd8 18.Cd5 Ae7 19.Ab5!? [19.Dc7+! Re8 20.Ab5 Ta8 21.Cxe7 Rxe7 22.Dxd6+ Re8 23.e6 fxe6 24.Dxe6+ Rd8 25.000 Dxf4+ 26.Rb1] 19...Txb5? My opponent goes for the only line where he has a trick. [19...Tb7+-] 20.Dc7+ Re8 21.Dxc8+ Ad8 22.Cc7+ Re7 23.exd6+! [23.Cxb5?? That's the trick!. 23...Aa5+] 23...Rf6 24.Dxd7 Dxf4 [24...Tb4 25.Cd5+; 24...Txb2 25.Ad4#] 25.Dxb5 Rg7 26.Dd3 10