Closed Sicilian Help!

willilo

Hi Everyone,

http://www.chess.com/games/view.html?id=731157#

Please view the link above. I really like this game and it's a model game for me to use as prep against someone for a coming tournoment but what I don't understand is what happens if white just takes black's g5 pawn on move 18.

 

Please help!!!

Analysis would be great!