Ivorian winger Gradel took season's tally to six as Leeds stretched their unbeaten league run to nine matches and climbed to within three points of the Londoners.
The home side took the lead in the 25th minute when Robert Snodgrass crossed from the right and Gradel slammed into the bottom corner from six yards.
The killer second arrived in the 70th minute when George McCartney sent Gradel scampering forward on the counter-attack.
The winger beat his marker and raced into the visitors' penalty area before burying an emphatic finish into the bottom corner.
Source: PA
Source: PA