The teen was able to use a trampoline to jump over the fence at the house where he'd been held for more than a year.
He ran to the gym, told employees he needed to hide and asked them to call police.
"He quickly came up and said 'Please help me, they're coming for me, help me. Can I hide behind here?'" described a gym worker.
"At that time we just thought that he was lost and dirty. You see he almost falls here. He had to pick up the chain on his foot. Then we realized this was more than just a lost child," described another witness.
Four people, including the teen's legal guardian at the time, have been convicted in the case.
They could each face up to 30 years in prison when sentenced.