Peschisolido pounced with 10 minutes remaining but on a night when the Blades discovered they will play Arsenal in the FA Cup semi-finals, their performance was distinctly average against a team who conceded six at Wolves on Saturday.
In-form striker Steve Kabba prodded United into an undeserved lead with his fifth goal in eight games just before the break before Andy Hessenthaler's team turned the game on its head with goals from Simon Osborn and Paul Shaw.
However, they could not hold on as the hosts stretched their unbeaten run in all competitions to eight games. But, on this form, Gunners boss Arsene Wenger will not be too worried when his team meet United at Old Trafford on April 13 as the Blades appeared slow and lethargic for much of the game and only wasteful finishing from the visitors kept them from defeat.
Kabba had a low shot blocked by the legs of Vince Bartram before opening the scoring, sweeping the ball home from close range following Michael Brown's downward header from a Michael Tonge corner.
Gillingham drew level after 52 minutes. Nayron Nosworthy skipped down the right and his low cross was allowed to run across the box before Osborn thumped the ball home from 12 yards.
Gillingham nosed in front with 21 minutes remaining when Nicky Southall's thumping drive from 25 yards was parried low to his right by goalkeeper Paddy Kenny and Shaw was on hand to net the rebound.
Mamady Sidibe should have made the game safe but another glaring miss let United off the hook and it proved costly after 80 minutes when Peschisolido turned smartly in the box to toe-poke the ball home from 10 yards following Tonge's low ball in from the left.