Swansea City missed the chance to strengthen their claim for an automatic promotion place after slipping to a slender defeat at Priestfield.
With Brentford losing and Huddersfield, Barnsley and Colchester all drawing, the Swans had the opportunity to make up ground on leaders Southend.
But a Darren Byfield penalty earned Gillingham a rare double of the Swans after Keith Lowe had brought down Matthew Jarvis in the 54th minute.
But despite the defeat, the Swans are still in the automatic hunt in fifth place.
Gillingham: Brown, Wallis, Cox, Sancho, Jackman, Black (Spiller 75), Pouton, Crofts, Jarvis, Mulligan (Harris 90), Byfield. Subs (not used): Crichton, Flynn, Cochrane.
Swansea City : Gueret, Tate, Monk, Lowe, Ricketts (Williams 61), Britton (Martinez 45), O'Leary, Robinson, Forbes, Akinfenwa, Trundle (Knight 61). Subs (not used): Way, Fallon.
Att: 6,909
Referee: A Marriner (W Midlands).
Swans boss Kenny Jackett made just one change to the side that earned an impressive 2-1 win over Brentford nine days previous, with Adrian Forbes rewarded for his powerful display ff the subs bench with a starting place instead of skipper Roberto Martinez in midfield.
Gillingham, battling against relegation despite their 3-2 win over Brentford in midweek, were forced to defend for most of the first-half as the Swans missed a string of chances.
Gillingham had the first chance, however, as Willy Gueret denied Tommy Black from close range after nine minutes.
Swansea responded immediately as Andy Robinson, influential throughout the half, forced former Welsh Under-21 'keeper Jason Bown into a fine save from 25 yards.
It sparked a host of opportunities for the Swans as Kristian O'Leary and Forbes came close.
Their best chance came just before the interval, however, as Robinson squared a pass to Lee Trundle, but his delayed shot flew straight at Brown.
The deadlock was eventually opened nine minutes after the break when referee Andre Marriner had no hesitation in pointing to the spot after Lowe brought down Jarvis.
There were no complaints and Byfield fired the Gills ahead from the spot, via the underside of the bar.
Jackett, who had already swapped Britton for Martinez at half time, sent on Leon Knight and Gillingham old boy Tom Williams for Trundle and Sam Ricketts.
But Swansea couldn't find a way passed a resolute Gills defence and they returned to South Wales pointless.
