After hearing from concerned residents and noticing the problem ourselves, we've been following the Racked report on a lack of gyms in Williamsburg with interest.
There is the Metropolitan Pool and Fitness Center, located on Bedford and Metropolitan. According to the Voice, it offers: "two gym rooms and a three-lane, 25-yard pool." Racked also got an email informing them of a new gym which will be opening soon on the south side near Graham Avenue or somewhere in the McCarren Park vicinity.
According to Yelp, there's a Curves, some personal trainers...and Frenchie's Gymnasium (which apparently looks like this). Anyone else have any leads on Williamsburg gyms?