g2-g4 in the opening

      
Is g2-g4 a important move for white to play in the opening? If so, is there a best moment to play g2-g4? 
Should white play g2-g4 before castle? Or is it best to play g2-g4 after castle?
   
1.g2-g4
A little too early, even if it was interesting to look at games played by IM Michael Basman and read his book The Killer Grob.   


2.g2-g4
Too early. Gambits like 1.d4 f5 2.g4 and 1.d4 Nf6 2.g4 has been played. Nothing that scares a Grandmaster.


3.g2-g4
Probably still too early. 


4.g2-g4
It starts to get interesting, but probably still too early.


5.g2-g4
Interesting, but probably still a little too early. 1.e4 d6 2.d4 Nf6 3.Nc3 g6 4.Be2 Bg7 5.g4 has for example been played. One game is Kovacevic - Seirawan, 1980 


6.g2-g4
Now it becomes really interesting. Not sure it's the best moment...
But white has won many games with a 6.g2-g4: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.g4 (Game collection) 


7.g2-g4
A little better moment than 6.g2-g4? Maybe not. 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.Be3 a6
7.g4 e5 8.Nf5 h5 (Game collection)
Another opening with a 7.g4: 1.d4 d5 2.c4 c6 3.Nc3 Nf6 4.Nf3 e6 5.e3 Nbd7 6.Qc2 Bd6 7.g4 Black should be ok. (Game collection)


8.g2-g4 
Black looks ok after 8.g4 in the following lines: 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.Be3 a6 7.f3 b5 8.g4 b4 9.Nce2 (9.Na4 Bd7 Game collection) 9...h6 (Game collection) 


9.g2-g4  
Black can play 8...b4 and stop white from playing 9.g2-g4:
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.Be3 a6 7.f3 b5 8.Qd2 b4 9.Nce2 (9.Na4 Nbd7 Game collection) 9...e5 (Game collection)
 
If you want to castle before playing g2-g4:
 
14.g2-g4 or 15.g2-g4
Not unusual for white in the Sicilian Dragon to wait with g2-g4 this long:
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 g6 6.Be3 Bg7 7.f3 O-O 8.Qd2 Nc6 9.Bc4 Bd7 10.O-O-O Rc8 11.Bb3 Ne5 12.h4 h5 13.Bg5 Rc5 14.g4 (Game collection) 

1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 g6 6.Be3 Bg7 7.f3 O-O 8.Qd2 Nc6 9.Bc4 Bd7 10.O-O-O Rc8 11.Bb3 Ne5 12.h4 h5 13.Bg5 Rc5 14.Kb1 b5 15.g4 (Game collection)
 
A 15.g2-g4 in the Sicilian Scheveningen:
1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 e6 6.Be2 a6 7.a4 Nc6 8.Be3 Be7 9.O-O O-O 10.f4 Qc7 11.Kh1 Re8 12.Bf3 Rb8 13.Qd2 Bd7 14.Nb3 b6 15.g4 Black looks ok. (Game collection)