This video gives some insight into black's ideas
Chess Opening for Black: The Gurgenidze System
a well known idea is to exchange the c8 Bishop with the help of h5 when white goes for f4 and then e5 after black plays d5
1.e4 g6 2.d4 Bg7 3.Nc3 c6 4.f4 d5 5.e5 h5 6.Nf3 Bg4 7. Be3 Nh6 8. h3 Bxf3 9. Qxf3 Nf5 10. Bf2 h4 11. Bd3 e6
The g7 ...