I think 16.gxf5 was a good move. 

16.b4 is possible but after 16...fxe4 17.dxe4 the pawn c4 might become a target after a later Nd8 and c6. 

After 16.gxf5 gxf5 gives 17.b4 White an easy game (+=/=) but that doesn't mean that your 17.d4 was a mistake. Your first real error in the game was 21.Rd1 IMO. Instead of this you could play 21.Rc1 when 21...Ba6 would be bad because of 22.Rfd1 when White can follow up with b4, a4 and so on. I think Black should play (21.Rc1) a5 22.Rfd1 Rd8 with a sharp position. Many players would be tempted to play 23.c5 then but after 23...d5 Black has much time to launch an attack along the g-file. I would rather play 23.a3 and 24.b4 (23...a4?! 24.Qb4) with a nice position. 

If I am not very mistaken you could claim a draw by threefold repetition at move 35 by intending to play 35.Qe1.