The Kingston Falls YMCA is the local YMCA located in the town of Kingston Falls. Among other sport-related things, it has a large swimming pool.
At the start of the night the gremlin invaded the town, Billy Peltzer along with Gizmo (being in Billy's backpack) tracked the lead gremlin Stripe to the YMCA and entered. Inside, as Billy looked for Stripe, the evil gremlin lunged out at him from a hiding place, spooking and clawing him before jumping into the swimming pool and started to multiply, making the water boil and emitting smoke. Billy backed off before fleeing, knowing what was about to happen.As he ran outside and away from the building, loads of new gremlins with Stripe their leader were flocking inside and heading for the exit.