Evan's Gambit Question

I have used the Giucco Piano many times, and recently added the Evan's Gambit, a common enough opening.   Although the defense is generally predictable, I played a friend the other night and he made a move that destroyed my attack, and really made me fumble around.  After he made the move it looked like something I would do as a non predictable defense, and I will use it :).  My question is:  Usually players I face, when White takes pawn at Q4, Black would normally retreat his bishop to B-Kt3.  In this case Black advances his Queen instead. 

What wrong with BG5?

Nothing wrong with that, thanks, its why I asked :).  I often seem to make the mistake of playing defensively against a Queen attack, and therefore do not consider counter attacking the Queen, it is something I need to work on.  Thanks again.