The 36-year-old, who played for the Millers seven seasons ago, latched onto Danny Kedwell's flick from a long Chris Whelpdale throw for the sole strike in the 22nd minute at Priestfield.
Gillingham now sit 11 points clear of fourth-placed Cheltenham and 12 ahead of Burton, who are fifth and have one game in hand like Port Vale in second and Northampton in third.
Burton's goal in the first half was a rare moment of note however after the break Gillingham could have extended their lead while Rotherham could have snatched a point.
Millers goalkeeper Scott Shearer tipped a 12-yard effort from Kedwell onto the bar in the 70th minute and then dived low to push a composed header from the same player wide moments later.
And right at the death Stuart Nelson had to be alert to save a powerful 35-yard strike from Michael O'Connor as the Gills also moved one step closer to the League Two title.