I use Club Metro on Washington St. I'm usually the only person in the gym wearing a mask. I go during the afternoon (between 12-3, usually) avoiding the rush. It's never crowded (though the dumbbell area is tight, so even a few people fill up the space).

They don't use any mitigation measures that I know of, but it is a large space with high ceilings, so there's that. It's infinitely less crowded than Base Gym. I've never had anybody look at me askance for masking.

It's not a fancy gym; a lot of the equipment has seen better days. But it's got everything you need and the price ($50/month) is fair for this part of town.