Goals from Brent Sancho and a Darren Byfield brace either side of half-time eased the Kent side's relegation worries.
Gillingham took the lead after nine minutes. Tommy Black's far post corner eluded everyone apart from an unmarked Sancho who headed home from two yards.
The lead barely lasted five minutes before the Gills returned the favour when they allowed Michael Turner acres of space to thump in Paul Brooker's right-wing corner.
Both sides continued to create chances. Byfield missed two presentable opportunities for the home team, while at the other end Jay Tabb came close to giving the Bees the lead on 29 minutes when his shot from a tight angle hit the post.
On the stroke of half-time Gillingham once again edged in front after a moment of madness from Sam Sodje.
Byfield's shot appeared to be going out of play but the Brentford centre-back brought down Matt Jarvis just inside the area and Byfield sent Stuart Nelson the wrong way from the spot.
The away side were given a lifeline when Alan Pouton was sent off for taking out Tabb who was through on goal on 72 minutes.
But Byfield extended the Gills' lead only two minutes after the sending off.
Spiller's through ball split the Bees defence and Byfield's shot was too powerful for Nelson to stop.
The game continued to swing back and forth and Brentford pulled a goal back on 78 minutes.
Lloyd Owusu's headed flick on caught the Gillingham defence napping and Tabb made no mistake with an exquisite volley.
Brentford piled on the pressure in the closing stages before Sean Clohessy was dismissed in injury time for a reckless challenge.