Watt opened the scoring with his first goal since October 2012 when he latched on to Brian Wilson's cross to convert from close range after 50 minutes.
Morrison doubled the hosts' advantage three minutes from time as he fired in a low strike from the edge of the area for only his third goal in 52 appearances.
Substitute Sears completed the scoring in stoppage time with a similar effort that clipped the post before crossing the goal-line.
On-loan defender Luke Garbutt tested Gills goalkeeper Stuart Nelson on several occasions, most notably with a powerful effort from the edge of the area midway through the first half, and again with a swerving shot from 35 yards on the hour-mark.
Wilson's inswinging effort was cleared off the goal-line by Leon Legge and Dominic Vose fired just over the crossbar from close range.
Cody McDonald had Gillingham's best chance in the first half with a 20-yard effort that went narrowly wide, while Danny Kedwell headed over the crossbar from Jake Hessenthaler's corner.