An early g4 in the QGD is a nice suprise for black in this opening. The idea itself is known from other openings (Keres attack), but I never saw it in the QGD at move five

[Event "Sparkassen GM"]
[Site "Dortmund GER"]
[Date "2010.07.24"]
[Round "9"]
[White "Mamedyarov, S."]
[Black "Ponomariov, R."]
[Result "1/2-1/2"]
[ECO "D37"]
[WhiteElo "2761"]
[BlackElo "2734"]
[PlyCount "85"]
[EventDate "2010.07.15"]

1. d4 d5 2. c4 e6 3. Nf3 Nf6 4. Nc3 Be7 5. g4 $5 {It isn't often that we give
a diagram for the 5th move of a game, but this is certainly deserving of one.
In a database with more than 37 thousand games with the position just before
g4, over 31 thousand had 5.Bg5 played, and fourteen (not thousand, just plain
fourteen) contained this move. All but one were by players of a *much* lower
standard unsurprisingly. The idea of an early g4 thrust is common in many
openings, and has even been introduced in QGD positions in the past, such as
Kasparov-Spassky (Barcelona, 1989), but never quite this early. Is this the
dawn of a new line, or will it just enter the annals of opening theory just to
illustrate GMs have tried them all? Only time will tell. In any case,
Mamedyarov clearly felt that in order to try and wrest the first prize from
Ponomariov, the moment was now, and he was ready to wrestle the bull by the
horns.} O-O ({It is tempting to prevent the advance of the g-pawn with} 5... h6
{but this is exactly the sort of reaction White is hoping for. It would weaken
Black's kingside, where he is bound to castle, and provides an attractive
target after} 6. Rg1) 6. g5 Ne4 7. Qc2 Nxg5 8. Bxg5 Bxg5 9. h4 Bh6 10. e3 c6
11. Ng5 f5 12. Rg1 Qe7 13. O-O-O Nd7 14. f3 {White is preparing a break with
e4 in order to open lines and renew his threats, while Black takes measures to
prevent it and complete his development.} Nf6 15. Kb1 Kh8 16. Bd3 e5 $1 {The
exclamation mark is not for positional reasons.} ({Black certainly had other
alternatives such as} 16... dxc4 17. Bxc4 b5 18. Bd3 a5 19. e4 g6 20. Rge1 $13
{But why enter an unclear line that is clearly double-edged, when he has no
need? The Ukrainian is leading by a point and has no need to take unnecessary
risks. The burden of proof is on Mamedyarov to show he can somehow win this,
so that being the case, Ponomariov's move, leading to simplifications, and a
much easier position to play, is by far the best practical decision.}) 17. cxd5
Nxd5 18. Nxd5 cxd5 19. dxe5 Qxe5 20. Qc5 Be6 21. f4 Qf6 22. Nxe6 Qxe6 23. Bc2
b6 24. Qxd5 Qxe3 25. Bxf5 Qc5 {After this, the queens come off, and Black is
now in safe waters.} 26. Qxc5 bxc5 27. Rd5 Bxf4 28. Rg4 Be3 29. h5 Bd4 30. Kc2
a5 31. a4 Rf6 32. Rg2 Raf8 33. Bd3 g6 34. hxg6 Rf2+ 35. Rxf2 Rxf2+ 36. Kd1 hxg6
37. Bxg6 Kg7 38. Bc2 Kf6 39. b3 Rf1+ 40. Ke2 Rf2+ 41. Kd1 Rf1+ 42. Ke2 Rf2+ 43.
Kd1 1/2-1/2

Source: Chessbase