The game got off to the worst start possible for the home side when, after just five minutes, Mark Saunders left the field with a broken leg after coming off second in a 50-50 challenge with Claude Davis.
It was the home side though who took the lead after just 15 minutes through the lively Darren Byfield.
He burst through the defence and picked up a fantastic through ball from Paul Smith before slotting the ball past Andy Lonergan.
Gillingham could have gone two up when Marlon Broomes almost scored a spectacular own goal as he mis-hit his clearance from the edge of the box and nearly managed to chip Lonergan.
Preston played themselves back into the game as the half went on and on the half-hour mark Chris Lucketti was unlucky not to equalise for the visitors.
The captain headed Alexander's corner goalwards only for John Hills to clear the ball off the line.
Minutes later the visitors were level through Ricardo Fuller, who returned from America only yesterday after visiting renowned knee specialist Richard Steadman.
Paul McKenna picked up a loose ball in midfield and played through a perfectly weighted ball for Fuller.
The Jamaican turned well to beat Chris Hope and, after one touch to steady the ball, fired the ball into the back of Steve Banks' net.
The home side secured all three points when summer signing Roberts headed home Danny Spiller's cross with just five minutes to go.
Gillingham's new strike-force both scored again and, as Leeds found out mid-week, it is turning out to be one of the more lethal in The Championship.