What would you do in this situation? Another child hurt my child and it’s been made out to be my child’s fault

Another child hit him, he didn’t retaliate and he is the one being blamed? No that isn’t fair.

Id be telling the gymnastics teacher exactly what happened and question have they ever seen your child do something before - no! So why do they think you’d be lying and him do something so out of character. I’d also be saying that you hadn’t intended on raising what happened to your son to avoid drama for the club but you feel you need to correct them and your upset at the accusations. I’d also personally get on my high horse and say why is there a presumption a boy would hit a girl but not a girl hit a boy - just because I’m a pain in the arse but it does make them think because there’s still such a different approach with genders and kids and their approach when tackling stuff like this, I’ve noticed girls get off lighter than boys would.
If this little girl has lashed out at your boy, the chances are she will again at another child and when she does, you can say I did warn you. Then be the bigger person and then let it go.

What did the other people say they saw?

Why would you not have spoken to the granny? She obviously sees it as her granddaughter was playing and your son came to her, maybe trying to take things off her? What do the teachers think happened, and who’s the people that saw different things?
They are 3, don’t overthink it, just keep a close eye and next time speak up.