Seven games now without a win has dumped them into the bottom four and the trap door is open with the threat of a second successive relegation.
History suggested a better outcome for they had finished as winners on their last three visits to Ashton Gate, but within the first 20 minutes of this game they were history.
A diagonal ball by Liam Fontaine from deep in City's half caught their backline flat-footed in the 11th minute.
As it landed Stephen Brooker turned central defender Ian Cox and calmly placed a shot inside the far post for his 14th goal of the season.
He might easily have gone off with a half-time hat-trick as Gillingham had little idea of how to stop him.
They had even less idea of how to stop the second goal following Alan Pouton's trip on Bas Savage.
Centre-back Louis Carey began a long run as Alex Russell took the kick and the ball dropped perfectly for him to claim the goal.
The third came from the penalty spot after Leon Johnson brought down Dave Cotterill. His tackle from behind brought him a red card.
City couldn't quite decide who should take the penalty but eventually Cole Skuse stepped up to send Jason Brown the wrong way.
Gills manager Ronnie Jepson immediately sent on Danny Jackman to take Johnson's place, sacrificing his striker Neil Harris.
It signalled the hope of a damage limitation exercise but he relented at the break, sending on Matthew Jarvis in the hope he would run at defenders.
Facing a superb Carey at the top of his form in the home defence, it was always a daunting task made worse when Carey went up for Russell's 50th minute corner and angled in a perfect header to score twice in a match for the first time.
The rout was completed by substitutes Luke Wilkshire and Mark McCammon.