But League One leaders Leeds fell well below their best as they made it 20 home league games without defeat.
Gradel, on loan from Leicester, replaced on-loan striker Sam Vokes soon after the hour as manager Simon Grayson tried to inject some life into his team and the substitution eventually paid off.
With time running out, another substitute, Aidan White, picked out Gradel inside the penalty box and the winger turned cleverly to lash the ball in from 12 yards on the angle.
Orient had the better of the first half, with Leeds' Casper Ankergren the busier keeper.
The Dane was beaten after only five minutes but Sean Thornton's tapped free-kick to Andros Townsend was taken too quickly for referee Dave Foster's liking and as Thornton's shot arrowed into the top corner the goal was ruled out.