Goals from Gary Mulligan, Adam Miller and Leroy Griffiths sealed an impressive win for the Gills who will end 2007 on a high.
Gillingham had begun brightly but were indebted to their goalkeeper, Simon Royce, for some fine saves including a smart stop to push away Sammy Clingan's stinging 25-yard drive in the 20th minute.
But the save of the half would be made by Royce's opposite number Paul Smith, who showed great reactions to push away Mulligan's close-range shot via a deflection off Luke Chambers.
Forest began to look dangerous but on 37 minutes Chris Cohen was guilty of woeful finishing, slicing a volley wide from ten yards after Junior Agogo's neat lay-off.
Colin Calderwood's men missed a golden chance to go in front just after the break when Skipper Ian Breckin flashed a header just wide from Kris Commons' free kick.
That proved to be the turning point as a minute later Gillingham went ahead through Mulligan, who tapped home from a yard out after Smith spilled Andrew Crofts' deflected long-range drive.
Royce then excelled himself again to deny Cohen with another fine stop and his side was celebrating again on 61 minutes.
Forest failed to properly clear Delroy Facey's cross, allowing Miller to drill home an unstoppable 25-yard drive into the bottom left-hand corner.
Forest threw on Matt Thornhill as an extra striker but it was Agogo who missed an absolute sitter on 70 minutes, heading wide Commons' perfect in-swinging cross from only six yards out.
It proved to be a crucial miss as Gillingham grabbed a killer third in the 81st minute through substitute Griffiths, who latched on to Kelvin Wilson's weak back header to lash home his first goal for the Club.