Two goals from Jordan Obita helped Gillingham to an impressive victory over promotion-chasing Cheltenham.
The Robins, who have now collected one point from four matches, had a chance to go ahead but Russell Penn miskicked from Jermaine McGlashan's pull-back.
Obita put the Gills in front when he held off Sido Jombati and shot low past Cheltenham goalkeeper Jack Butland.
A defensive error saw Obita lob Butland for 2-0 before Chris Whelpdale sealed it, tapping home Danny Kedwell's cross.
Cheltenham were top in February but are now fifth, while Gillingham remain one point and one place off the play-offs in eighth.
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Cheltenham Town manager Mark Yates told BBC Gloucestershire:
"It was our worst performance of the season by a country mile. I apologise to the fans - it was nice to see them turn up at the end when they started booing.
"It was quiet for 35 to 40 minutes, then they score and their fans are up. I'm asking for noise and for them to get behind the players.
"I must have picked the wrong team. Not one player can be anywhere near happy with their contribution - our best player [Sido Jombati] could have cost us two goals. The first goal has changed the game. They've comprehensively beat us, I have no problem with that, but all three goals are poor on our part.
"We've got nine games left and we've got to get to where we were last week in terms of performance. It's about attitude now. We made poor decisions with and without the ball. We've lost three on the spin and we have to address that quickly."
Gillingham manager Andy Hessenthaler told BBC Radio Kent:
"It's right up there, to come to Cheltenham, with the season they're having, to score three and keep a clean sheet says it all.
"The dressing room's so together and we really want to achieve something.
"They were exceptional, everyone was terrific, we limited them to maybe one shot on target.
"We could have had more, to come and do that at a place like Cheltenham is a terrific achievement."