The Gills carved Oxford open several times in the first half but they were denied by recalled U's goalkeeper Ryan Clarke, who pulled off a series of top-class saves.
In the second half, however, United more than held their own and Alfie Potter went close with a lofted shot which Stuart Nelson pushed over the crossbar.
Clarke made a fine double save in the third minute from Danny Kedwell and Adam Barrett. Chris Whelpdale then planted a glancing header wide, while Barrett's header from a corner was cleared virtually off the line by Jake Forster-Caskey.
Clarke saved superbly from Deon Burton either side of half-time but then the home side came much more into the game and substitute Tom Craddock went close with a shot which was deflected over.
It was Oxford's first clean sheet in nine games following their sorry September, while the Gills saw their lead at the summit cut to three points.