In-form Biggins popped up in the box on the hour mark to meet Kyle Knoyle's cross and head past Glenn Morris, having earlier hit the bar with a free-kick from 30 yards.
Visitors Gillingham offered very little in attack but did see Max Ehmer's header tipped onto the bar with the score at 0-0.
Scroll down for out player ratings.
1. Match action
Harrison Biggins celebrates his goal. Photo: HOWARD ROE, Howard Roe/AHPIX LTD
2. Jonathan Mitchell - 7
Got caught dallying in possession with Ro-Shaun Williams but made a few saves to keep Gillingham at bay, most notably tipping a header onto the bar. Photo: Howard Roe/AHPIX LTD
3. Kyle Knoyle - 8
Offered a constant attacking threat down the right-hand side and assisted the opening goal. Photo: Howard Roe/AHPIX LTD
4. Adam Long - 7
Largely solid defensively and looked OK in possession, too. Replaced due to injury. Photo: Howard Roe/AHPIX LTD