I'm somewhat familiar with the Evans, but not so much with Qg5.
Well, 11. ... Qg5 threatens the old 1-2 queen pin. The basic idea is 12. ... Bh3 threatening mate, forcing 13. g3 Bxf1 which weakens the king position and wins an exchange.
Personally I feel like 12. Bc1 Qh5 13. Be3 would be just a bit safer, but anyway..
White plays 12. Qxc6. Black wanted to play Bh3 and grab the exchange, if Bh3 now, the rook is protected but not the knight. So, black needs to be doing something else with the bishop to protect both pieces and 12. ... Bf5 should do just that.
Black's material is rather loose at white's 13th move. Here's a bit of puzzle for you, white to play and win a pawn.
**Disclaimer: It wins a pawn, but it also adds about a pawn's worth of trouble for white. It's not necessarily the best move

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'Solution' below
click to show
13. Bxf7+ Kxf7 14. Qd5+ Be6 15. Qxe4