This move order, without e4 and Bg7, was employed by Botvinnik in the Tal-Botvinnik 1960 WCC match. If you're interested, check it out here. In his book on the match, Tal recommends going for a quick h6-g5-Nh5 maneuver to win the bishop pair before the kingside knight can be maneuvered to c4, which (in conjunction with the bishop on g3) puts uncomfortable pressure on your backwards d6 pawn. Tal also mentions that while it looks a little weird to weaken your kingside pawns so much so early on, any attack on your king is merely illusion as your pieces are well coordinated to compensate for the few light square weaknesses.
With your specific move order, however, you're going to want to insert 9...a6 before playing g5 so as to stop the following variation: 8...h6 9. Bh4 g5 10. Bg3 Nh5? 11. Bb5+! and you cannot comfortably block the check without losing your d6 pawn. Note that Bd7 "loses" to Bxd7+ Qxd7 Ne5! with a counterattack on your undefended h5 knight.
Therefore, the recommendation according to theory is:
1. d4 Nf6 2. c4 c5 3. d5 e6 4. Nc3 exd5 5. cxd5 d6 6. e4 g6 7. Nf3 Bg7 8. Bg5 h6 9. Bh4 a6 (9... g5 10. Bg3 Nh5? 11. Bb5+! Kf8 12. e5!) 10. a4 (10. Nd2 b5!) g5 11. Bg3 Nh5 12. Nd2 Nxg3=