Simeon Jackson scored both goals to make it 20 for the season and fire Gillingham to Wembley.
Jackson kept his cool to fire home a second-half penalty and secure a 2-1 win for the Gills in the League Two play-off semi-final second leg against Rochdale.
The Canadian had put the Gills ahead in the first half, but his goal was cancelled out by Chris Dagnall's strike just before the break.
But Rory McArdle brought down John Nutter and Jackson - who sent his last penalty over the bar at Wycombe last month - scored the goal to set up a play-off final at Wembley next Saturday against Shrewsbury.
The Gills were sloppy in possession early on, but they did muster the first effort on goal when captain Barry Fuller, yet to score for the club, tried his luck from 35 yards - but it was over the top.
Dale had their first chance when Tom Kennedy crossed for Lee Thorpe, but he headed high over the top - and the Gills went ahead in the 13th minute.
Dennis Oli won the ball off Rory McArdle and despite the ball accidentally striking his hand, played in Andy Barcham on the left. He crossed low and Jackson stole in front of his marker to net at the near post.
Dale levelled against the run of play in the 37th minute when Fuller's clearance was charged down and fell to Dagnall, who cut inside Nutter before finishing left-footed.
The Gills were awarded a penalty just after the hour when Barcham cleverly flicked in Nutter and he was brought down by McArdle, and Jackson stepped up to send Frank Fielding the wrong way.
Rochdale manager Keith Hill responded by throwing on strikers Adam Le Fondre and Jon Shaw, and their presence almost forced an error as Royce made a hash of a corner, but he recovered well to cling on at the second attempt.
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