Oli rose highest to head home John Nutter's in-swinging free-kick with two minutes remaining to end the Gills' four-game winless streak.
Sam Saunders' strike just before the break looked to have earned the Daggers a well-deserved point after Mark McCammon had given the Gills an early lead.
Still said: "We're disappointed with the result. With the greatest respect to Gillingham, I think they got away with it.
"I thought we deserved something from the game. From the moment they scored, we were the better team. We worked hard, I can't remember Rob (Tony Roberts) having much to do and I thought their goalkeeper looked unbelievably edgy.
"His kicking was poor, he put his back four under pressure, and that made them uncertain. We would have been happy with a draw, but even at 1-1 we were pushing men on because we want to win games."