With Simon Grayson's squad currently short on signings and strikers, they looked to be heading for a first defeat of the summer thanks to a first-half goal from their former midfielder David Prutton on Saturday.
But Gradel ensured United remained unbeaten, if not convincingly, when he sent Nicky Weaver the wrong way with seven minutes left after he had been squashed by Reda Johnson in the box.
It was all Wednesday in the opening half and, after Chris Sedgwick headed against the bar, Prutton put them ahead when he ran past four players before angling a drive across Paul Rachubka with 10 minutes on the clock.
Leeds lost injury-prone defender Ben Parker to another problem not long after, and Johnson and Jermaine Johnson both wasted opportunities to put Leeds out of sight.
But the introduction of Ross McCormack gave Leeds some life and, after Gradel and Adam Clayton laid on a move that ended with Lloyd Sam hitting the post, Gradel himself levelled things up.