The crowd was over 5,500 more than saw the Whites win the old First Division title in 1992.
But the match didn't live up to the party atmosphere in the first half. Gary McAllister rested several key players and made some surprise positional changes, including Bradley Johnson at left-back.
Defender Paul Huntington gifted Gillingham's opener, smartly taken by Simeon Jackson, and the visitors, playing five at the back, but keeping two up front, were the more positive side in the first period.
Gillingham were gradually pushed back after the interval and were rocked by Johnson's blistering 69th-minute volley after great work by substitute Seb Carole.
Needing victory to have any chance of avoiding the drop, the Gills threw men forward with abandon towards the end and were inevitably caught on the break.
Andrew Hughes' long crossfield pass was taken on by Tresor Kandol, who scored the winner two minutes from the end of normal time.
It still needed the woodwork to prevent a Nicky Southall injury-time equaliser, but no one was going to stop United's long-suffering fans' enjoying their day in the sun.
Leeds now head to the League One play-offs in buoyant mood after finishing fifth while defeat relegated the Gills.
Carlisle, managed by ex-York City boss John Ward, now stand between Leeds and a trip to Wembley. The play-off semi-final first leg will be at Leeds on Monday, May 12 (7.45pm) and the return at Brunton Park on Thursday, May 15 (7.45pm).