Best practical chances after g5 forking knight and bishop I am a beginner playing white and it's my turn to move. What is the best possible move in response to this fork?

 [title "Game position after fork:"]
 [fen "r1bqr1k1/ppp2p2/2n1pn1p/3p2p1/1b1P1N1B/2N1P3/PPPQBPPP/R3K2R w KQ - 0 1"]
Disclaimer :  in any case you are lost if your opponent plays reasonnably enough.

Assuming that you don't want to resign at move 10, my suggestion would be to continue with 10.Bg3 gf4 11.Bf4. Black's king is slightly weakened, you hit h6 and plan to continue with 0-0-0,f3,g4,h4,g5 to try and open his kingside as much as possible. In positions with opposite castles, material is not the single most important criteria, king security and initiative matter too.

If Black plays passively or conservatively, say 11...Kg7, then 12.f3 prevents Ne4, and with Bd3 and g4-g5 you may be able to muddy waters enough to induce later mistakes by Black.

If he plays actively and purposefully, with 11...Ne4 (or even 10...Ne4) 12.Qd3 e5!, not worrying about the Ph6 and intending ...Bf5, well, you hardly have anything to hope for...