Conor Coady scored his second goal in four days as Sheffield United won away from home in League One for the first time since November 23.
The on-loan midfielder fired home from close range shortly before the break to give United a deserved victory at the Priestfield Stadium.
Blades manager Nigel Clough made four changes to the side who came from behind to beat Nottingham Forest in the FA Cup on Sunday.
However, Ben Davies, who signed on loan from Derby County yesterday afternoon, had to be replaced in the starting line-up by Bob Harris after getting stuck in traffic en route to the ground.
The hosts started brightly, Mark Howard saving well to deny Connor Smith in the fifth minute, and debutant defender Ryan Inniss fired across the face of the goal from close range shortly after.
After scoring twice in the defeat of Forest, Chris Porter believed he had put the Blades in front on 21 minutes from Harris’s inswinging cross, but he was adjudged to have been offside.
He then hit the top of a post four minutes before the break, but after the Gillingham defence failed to clear, Coady hit a low shot past Stuart Nelson for what would prove to be the game’s only goal.
Harry Maguire, left unmarked, nearly doubled the Blades’ advantage 10 minutes into the second half, but Gillingham captain Adam Barrett produced a vital block to stop the defender’s goal-bound volley.
Billy Paynter went even closer on 62 minutes, but his low shot was brilliantly saved by Nelson.
Adebayo Akinfenwa went closest for the hosts, who had won their previous three games at home, but his header landed on the roof of the net.
Gillingham: Nelson, Martin, Barrett, Inniss (Legge 42), Harriman, Dack, Gregory, Linganzi (Weston 73), Smith (Whelpdale 64), Akinfenwa, McDonald. Unused substitutes: Baxter, Hessenthaler, Fagan, Pigott.
Sheffield United: Howard, Hill, Collins, Maguire, Harris, McGinn, Doyle, Coady, Flynn (Davies 73), Paynter (Murphy 69), Porter (Baxter 79). Unused substitutes: Long, Kennedy, Baxter, Scougall, Miller.
Referee: Brendan Malone (Wiltshire)