The 13 sites in Scotland and three in Wales will follow after local restrictions are lifted, the group said.
READ: The Gym Group restarted at ‘overweight’ as Barclays says risk/reward profile is “sufficiently attractive”
The initial opening hours will be 6am-10pm on weekdays and 8am-8pm on weekends, but the firm expects to return to 24/7 operations within the next few months after trials in certain sites.
Venues will operate under strict safety protocols including temperature checks for staff, hand sanitiser stations available across the rooms and restrictions over the number of people allowed in.
Customers can continue to freeze their membership for free or use a discounted offer for at-home classes with fitness app Fiit.
The firm saw a significant number of cancellations in the first days after closures in March but said they slowed considerably thereafter.
As of July 9, Gym Group said it had 692,000 members against 870,000 on March 18.
"The Gym Group will continue to benefit from the long-term trend in health and fitness and its low-cost offer should be well-suited as the economy recovers," analysts at Liberum commented.
Shares rose 2% to 152.53p early on Friday.
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