The injury-hit Black Cats toiled for a winner after Chris Brown's fourth goal of the season had hauled them back into the game following Darius Henderson's surprise opener for the visitors.
But they could not seriously test Gills goalkeeper Steve Banks and it was Stan Ternent's team who could have snatched it in the end.
The game got off to a scrappy, low-key start and it was Gillingham who got the first effort on goal in the seventh minute, Henderson hooking a tame effort straight at Sunderland goalkeeper Thomas Myhre from only eight yards out.
An underhit back pass from Dean Whitehead caused a flutter in the Sunderland rearguard as Darren Byfield closed in on Myhre. The striker blocked the keeper's attempted clearance, but shot disappointingly wide from the rebound.
Sunderland had the ball in the net through Sean Thornton in the tenth minute, but he was flagged offside.
Gills went in front as confusion in the home defence let Byfield clear on the left and his excellent cross was met by Henderson's deft header, which flashed in past Myhre.
The advantage lasted only four minutes however as Sunderland struck back. George McCartney had been a menace to the visitors in the opening half, regularly joining attacks, and he created the equaliser.
The Northern Ireland international skipped past three men on the left and crossed low to the near post where Brown prodded home in the 18th minute.
Henderson went close to a second just before the half-hour, his downward header being held well by Myhre low at his right-hand post.
Brown volleyed wide after a good move by Sunderland in the 33rd minute, but the home crowd's frustrations were growing and they made their dissatisfaction clear at the break.
Sunderland pressed for a winner after the break, but Whitehead was denied by Banks in the 49th minute from a good Thornton delivery and the Gills keeper did well again in the 56th minute to hold Brown shot from the edge of the box.
Black Cats boss Mick McCarthy sent on Matt Piper and Michael Bridges in a bid to find the breakthrough, but it was Gillingham who spurned a great chance in the 72nd minute as the lively Matthew Jarvis broke down the left and centred for Byfield, who completely fluffed his effort from only eight yards and Myhre gathered.
The home side went close with ten minutes to go as a scramble in the Gills defence saw the ball fall to Whitehead on the edge of the box, but Banks kept out the midfielder's volley.