Why wouldn’t black attack g5 with 7h6 instead of be7 in queens gambit?

Diagram shows the more detailed answer. In short you either lose a pawn or you have a terrible pawn structure on the kingside.

#3
Fully agree.
In addition two other points.
Later in the game black can kick the bishop with ...h6, but if it is good or bad is debatable. ...h6 forces the bishop to trade or retreat, and ...h6 makes an escape hole for a king on g8, but ...h6 also weakes the position and allows attacks based on that.
Black is not forced to play 4...Be7 and can also ignore the pin 4...Nbd7 5 cxd5 exd5 6 Nxd5? Nxd5 7 Bxd8 Bb4+ black wins.

You're welcome.

@tygxc

Good points. Myself, I play Slav against 1.d4, but I think I've seen this trick before (where black gains a piece). Either in some video on QGD or in some other opening, as I wouldn't be surprised if it can occur somewhere else as well.

In some QGD variations it is better for black to play h6. One such example is Tartakower's, where black will play b6. In such positions, it's good for white to play cd. It's even better for white if black has no knight on f6 to recapture with, so in this variation, black never plays b6 and then h6. It's always the other way round, to test whether white will immediately play Bxf6 or not.

In the Capablanca Variation, however, it's bad for black to play h6 before ....dc and ....Nd5, because the main point, to swap off white's bishop on g5, is then lost, if the B has retreated to h4. Then it can't be forced off and black's opening strategy has failed. That's a mistake often made by beginners.

Incidentally, h6 isn't weakening if black knows what he's doing. Very often black might play Re8 and Nd7 - f8. Then black has two knights guarding h7 and the knight on f6 is free to join in tactics elsewhere on the board. This is a difficult set-up for white to crack and white often has to make a diversionary queenside attack to tempt the knight out from f8 and set up the pieces so that white can switch back to the kingside faster than black. Then black may have to play g6 and with h6 already having been played, white may be able to sacrifice a piece on g6 or attack g6 some other way. In such situations I try to play f2-f4 and Rf1 - f3. Then black very often is in a lot of trouble.

#7
Yes, here is a famous example with 6...h6
https://www.chessgames.com/perl/chessgame?gid=1049648 

That was well played by Geller. White shouldn't play such desperation tactics as to prematurely win that pawn. It's better for white to finish development.

#10
Geller had told Spassky about the move 14...Qb7 in their preparation, but Spassky had forgotten that during the 6th match game against Fischer in 1972.
https://www.chessgames.com/perl/chessgame?gid=1044366 

Yes, I could tell it was a prepared line and I immediately liked 14 .... Qb7.

I've moved back to the Tartakower as my prime defence to 1. d4, from the Modern Benoni, so I'll remember that. Thanks.