Can the model be franchised? Are your margins large enough such that a franchisee will be able to make a profit AND pay your franchise fees?
If you go down the franchise route, you will be changing the nature of your business from being a gym operator to being a franchisor, or seller of business systems.
Do you have your systems perfected to the point where someone will pay for them?
Do you have the resources to start supporting the needs of franchisees? What kinds of needs will they have? Coaching, support, etc. They will be expecting innovation in marketing and product delivery from you.
Are you prepared to sell off your existing locations as franchises and focus solely on this new business model for yourself?
In 2007 I owned a small business with a partner and faced the same question you are now. In the end we decided to sell our operating company and sell the IP within our systems in a 'do it yourself business kit.' It's worked brilliantly because we required no further investment but get a cash flow from the materials.
If you'd like an idea of how to format an attractive franchise opportunity. I recommend my recent book Franchise Warnings. Available from Amazon.