How do I handle GST and invoices as a contractor when my gym handles all the payments?

Starting business as an independent contractor for a gym. I sell my personal training service to customers, but all my client’s payments are supposed to go through the gym, and I invoice the gym for my cut of the sales – they keep a percentage (30%) as a “rent” fee. I’m struggling to understand how to apply GST in this situation as a small business owner.

I don’t have a GST number right now, and it’s my understanding I don’t necessarily need one until I reach $30,000 in profits. But the gym charges GST for its services. Do I charge GST on behalf of the gym taking the payment? Let’s say I do have a GST number, when I invoice the gym for my payment, do I ask them to pay me the full amount of GST I charged so I can remit that to the CRA? Or do I just ask for my 70% of the services sold and leave all GST with the gym?

Any help in understanding this would be very helpful.