I thought g3 ensure to capture the queen because with this move the black queen can't move on a position that is not "controlled" by a white piece.
But you're right I missed the "double threat". With this in worst case g5 ensure to win rook (risk to loose bishop). In worst case g3 win black queen but some position (f6) implies to loose white queen.
Thank you for your help I understand why g5 is better now :)
Um, I can't quite follow you. With "g5", do you mean g4 or Bg5? And in which variation does White lose his queen (as in: not just trade, like 20. g3 g6 21. gxh4 gxf5)?
Also, the black queen can go back to e7 after 20. g3, which is not controlled by a white piece. You can attack her once more with 21. Bg5, but that is parried by ...f6. This is what I meant when I said you were almost there (seeing that the bishop can attack both rook and queen on the d8-h4 diagonal, but in the wrong move order). Did you calculate this variation?