This might be the most realistic proposal for a somewhat safe-ish reopening scheme I've seen so far:
If a state or locality adopted this rule for work and school, gyms could piggyback on the schedule.
Opinion | 10-4: How to Reopen the Economy by Exploiting the Coronavirus’s Weak Spot (Published 2020)
People can work in two-week cycles, on the job for four days then, by the time they might become infectious, 10 days at home in lockdown.
www.nytimes.com
If a state or locality adopted this rule for work and school, gyms could piggyback on the schedule.