The King’s Gambit isn’t an opening in which the theory develops very quickly, largely because it is played so little. But once in a while something happens.
In my 2005 book, Play 1.e4 e5 I recommended the King’s Gambit Declined with 2…Bc5 as a sound and economical choice. But the following year David Vigorito mentioned the move 1. e4 e5 2. f4 Bc5 3. Nc3 d6 4. Nf3 Nf6 5. Bc4 Nc6 6. d3 Bg4 7. Na4 O-O 8. Nxc5 dxc5 9. O-O Qd6 10. Qd2!? as an interesting possibility. And a couple of strong players have played it.
In the following game Black finds an interesting counter in 10… Bxf3 11. gxf3 exf4 12. Qxf4 Qxf4 13. Bxf4 b5!?, the point being that after 14.Bxb5 Nd4, Black threatens both the bishop on b5 and 15…Ne2+. But 14.Bb3 is a good answer and Black would be struggling after 15.a4.
So this strange 10.Qd2 seems to be posing a problem or two, or at least producing an interesting game. But don’t count on it doing so for too long.