Why does 6.Bg5 prevent 6...e5 in the Sicilian Najdorf? In Gufeld's book, after 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Bg5!,

[FEN ""]
1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 6. Bg5!


he writes: "In this way White solves three problems: he prevents  ...e5, " but he shows nothing to support the statement.  Why is ...e5  so bad for black?
Gerard
The lines presented below are from the Chess Informant ECO 1984, but I doubt the assessments have changed since:

[fen ""]
[White "Sicilian defense"]
[Black "Najdorf variation"]

1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6 6.Bg5 e5?! 7.Bxf6 Qxf6 ( 7...gxf6 8.Nf5! Bxf5 9.exf5+/- ) 8.Nd5 Qd8 9.Nf5 Bxf5 ( 9...Be6 10.Bc4 Nd7 11.O-O Rc8 12.Bb3 Nc5 13.Qf3 Nxb3 14.Qxb3 Rc5 15.Rad1 Rb5 16.Qg3!+/- ) 10.exf5 Be7 11.c4+/- 


Although my edition of the ECO is old, I doubt anything has changed here.

In the main line Black has no counter-play-his only plan of attack-b5 is useless as White will take with the bishop thus eliminating every Black's counter-play. 

White has strong grip on the light squares, and presence of the bishops of the opposite color increases his advantage-remember this rule: 

Presence of the opposite colored bishops favors the side with the initiative. Although they may end up as a draw in the endgame, in the middle-game side that has an attack usually wins.

In the first sideline, Black tried to generate some counter-play with f5 but White's Qh5 keeps advantage. If Black continues with passive play then White keeps advantage due to the initiative and presence of the opposite colored bishops.

Black's last sideline features Be6 trying to ignore Whites plan, but we see that White develops strong pressure again.

I am not an 1.e4 player, so I was unable to find more concrete lines but these illustrate well enough why e5 is dubious move for Black. If you need better coverage then try to find some repertoire books-at this point I can not recommend any.

Best regards.

EDIT IN RESPONSE TO THE COMMENTS:

In comments, it was suggested that 7...gxf6 is playable, by applying the ideas from the Sveshnikov sicilian.

I am not an e4 player but let me try to explain why the above is not true.

First, let us see what the actual plans behind the Sveshnikov are. In order to preserve space and provide minimal yet sufficient info, I will quote Wikipedia article:


  8...b5! was Sveshnikov's innovation, controlling c4 and threatening
  ...b4 forking White's knights. Previously, Black played 8...Be6 (the
  Bird Variation), which allowed the a3-knight to return to life with
  9.Nc4.


These are the compensations Black gets, which are important to mention here, that eventually help him equalize:


  White's powerful knight on d5 and Black's shattered kingside pawn
  structure are compensated by Black's bishop pair and White's offside
  knight on a3. Also, Black has the plan of playing 10...f5, followed by
  ...fxe4 and ...f5 with the second f-pawn, which would give them good
  control of the centre. An alternative plan is to play 10...Bg7
  followed by ...Ne7 to immediately trade off White's powerful knight;


OK, this is enough for us to see why the line in the OP is bad for Black. Before we continue let us do a quick summary for the Sveshnikov:


White has horrible knight on a3 that Black harrasses with ...b5. This move prevents Nc4 and Bc4, indirectly fighting for the light squares and giving Black time to finish development/organize counterplay, since White is forced to reposition Na3 to a better square (not to mention Black threatens to win a piece by forking the knights with ...b4).
Black has bishop pair, which is good asset for the endgame
Black can destroy White center and establish himself there, with moves ...f5 + fxe4 + f5 


[Title "Black to move"]
[StartFlipped "0"]
[fen "rn1qkb1r/1p3p1p/p2p1p2/4pP2/8/2N5/PPP2PPP/R2QKB1R b KQkq - 0 1"]


In the above position the only thing identical to the Sveshnikov is the Black pawn structure with everything else different, and that matters as we shall see!


White has no Na3 which means that thematic ...b5 simply wastes a tempo and has no real strength as in Sveshnikov. 
Without Na3 and ...b5, White really has no coordination problems (no fork threats, all of white pieces are idealy placed) as in Sveshnikov, which makes his development and slight space advantage more dangerous.
Black has no way to establish central dominance as in Sveshnikov, because ...f5 + fxe4 + f5 plan is stopped by White pawn on f5
Black has no bishop pair, as in Sveshnikov
Black can not play d5, because he is late with ...Nc6 + Ne7, which leaves Bf8 passive since d6 + e5 chain is hindering him
Black will have problems finding safe place for the king
Black queenside play with ...b5 + Rc8 is weak, it lacks the effect it has in other lines of the Sicilian
White has no weaknesses, apart from f5 pawn, which can be sacrificed in some lines but also easily defended as well


White can find an effective plan here, simple computer analysis will do. I have no access to an engine here, but I believe that g3 + Bg2 plan is very strong here. White can attack everywhere. Any minor piece duel is advantageous for White ( Nc3 vs Bf8, Bf1 vs Bf8 if heavy pieces stay on the board which is very likely, Bf1 vs Nb8, even Nc3 vs Nb8). White also has advantage in heavy pieces only middlegames. White has better pawn structure and faster development.

Meanwhile, Black's only good piece could be Nb8, because it can swiftly go to d4. Still, White can consider exchange sacrifice and still end up better! Black really has nothing to attack, nor we can envision any plan that achieves any meaningful, long-term counterplay.

To conclude, after ...gxf6 Black gets worse version of the Sveshnikov, painful "fight-for-a-draw" position without any counterplay.