When the gym have to give me the incident report? What to do if the gym doesn't give me the incident report? Who is responsible for her injury?

Incident reports are protected work product that the gym does not have to disclose to you. Additionally, as my colleague pointed out, the gym is not vicariously liable for the intentional acts of one of their paying customers, unless they have reason to believe or knew that this child that hit your child had a propensity to commit violence. However, it is possible that you may have a claim against the child's parents for Negligent supervision, if they knew that their child had a propensity to commit violence and didn't take steps or warn the gym when their child used their facilities. How badly was your child injured from the attack?

Incident reports are generally legal "work product" and do not have to be produced. Also, in 99% of cases - facilities are not not legally responsible for the intentional acts of other children. If your daughter was hurt badly, you will need to retain a lawyer to examine whether there is some liability on an insured or recoverable party.