Mark McCammon scored a last-minute goal for Gillingham to earn them a first away win since January at Darlington.
Simeon Jackson had put the Gills ahead against the run of play when he picked out Simon Royce's goal-kick and fired low past Simon Brown at the other end.
Darlington looked to have salvaged a point thanks to Rob Purdie's penalty after 78 minutes.
But McCammon claimed the winner on the line as he touched in Garry Richard's back heel right at the death.
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