Is there any refutation to the Sicilian Grand Prix Attack?

I occasionally castle long as black but by and large I prefer to castle kingside and open the queenside to use it as a pivot to attack white's kingside. Occasionally I castle kingside and advance the g and f pawns, if the K is safe enough on h8. Sometimes black's K is safe in the centre, especially if pawns are still on f7, e6 and d7 or d6.

I don't really agree with your assessment, insofar as it fits (or doesn't fit!) what people actually play.

I've started playing 2. ... a6 against 2. Nc3 but I've been playing it against 2. Nf3 for at least 25 years. I'm starting to get good at it and have developed theory quite a bit.

Normally, the move order would be 1. e4 ... c5 2. Nc3 ...a6 3. f4, if white is determined to play the GPA. If black continues with b5, white can pressurise it with a4 and play to disrupt black's development. It can become quite a melee. However, I prefer a more principled development as black and would therefore play 3. ... e6, intending ... d5. White can no longer play the positional line with Bb5. This Bb5 started to appear more often 20 years ago in club and tournament circuits in the UK and I decided early on that it's strong for white in the hands of a strongly positional player. So I avoid it.

So white's development against 2. ...a6 with 3. g3 is actually a complete change of plan, where white makes the correctly principled decision that g3 is the best way forward, against an a6 Sicilian. It's no longer a GPA, although white may try to play f4 before Nf3.

I really like your position after 14. ...f5. Black is decidedly better. White immediately blunders, because he has to play 18. Qd2 and centralise rooks if possible. Bd2 is really bad.

@Optimissed I've given the lines with a4 as well as ...e6 and a quick ...d5 in the notes to the game above. I've also given black's plan against and early f4 by white (the key GPA move) in post #20.

Of course, I'm not a Polugaevsky or Zaitsev, so my opening suggestions need not be the best ever, but this particular line (at least to me) seems effective enough, especially on a psychological level.

I didn't notice anything much in post #20. You just said that you recommend ...b5. The idea with an early ... b5 is the method used by Kan. If white knows what he's doing, he will switch plans and attack the Q-side, when the only benefit of what black has played is really its surprise value. Black's Q-side is rather weak but it may be helped by the fact that white has played Nc3, which can be a tempo-target.

Tbh I don't think white has to change plans. White can play a4 and Bc4. Black's plan with e6 and Ne7 equalises and it's something that I've played. White can prepare for it with d3 and just drop back to Bb3. I would think the position is equal but black still has to be very careful. White's development is better than black's. Playing pawn on, d5-d4 can weaken black's position and that may be better avoided. When you come up against someone with white who deliberately plays for these positions, which isn't often, it can be a learning experience.