The bishop is attacked, so Black needs to waste another move instead of protecting the king.
1. g1f3  1... d7d5  2. g2g3  2... c7c5  3. f1g2  3... b8c6  4. c2c4  4... d5d4  5. e1g1  5... e7e5  6. d2d3  6... f8d6 Black chooses to develop with Bd6-Nge7. Other options are possible: Nf6-Be7, Bd6 with Nf6, and even Nge7-Ng6-Be7. In all cases the game is even. 7. e2e3  7... g8e7  8. e3d4  8... c5d4 The most traditional capture and also the best. 9. b1d2  9... e8g8  10. a2a3  10... a7a5  11. f1e1  11... c8f5 [%c_effect f5;square;f5;type;Inaccuracy;persistent;true] This move doesn't suit Black's development. 12. d2e4  12... a5a4  13. f3h4 [%c_effect h4;square;h4;type;Interesting;persistent;true] White bothers both Black's bishops and prepares the queen's journey to the kingside. 13... f5e6 A risky move, as it invites the knight to g5. 14. e4g5  14... e6d7 [%c_effect d7;square;d7;type;Mistake;persistent;true] The losing move. With the bishop on d6 unprotected, White's attack flows naturally. 15. d1h5 [%c_effect h5;square;h5;type;GreatFind;persistent;true] 15... h7h6 