In https://www.vox.com/21296067/coronavirus-covid-symptoms-supe... you will find the following passage indicating that community spread doesn't merely happen in gyms, but gyms are actually a prime location type for it to happen:

Researchers have been tracking many superspreading events around the globe, and there seem to be recurring locations no matter what the country. In addition to those we have heard most about, like prisons, food processing plants, and elder care facilities, there have also been numerous large superspreading events at bars, churches, offices, gyms, and shopping centers.

(Note gyms are on the list.)