The New Zealander managed to slot the crucial extra two points after replacement centre Ayoola Erinle slid over for a last-gasp try.
The victory puts Wasps within two points of league leaders Bath; the champions have now not lost in the Zurich Premiership since going down to Leicester Tigers at Welford Road on November 8, 2003.
It will be hard for Quins to recover from their last-minute reversal, but they must look at the positives – this was an impressive performance from a side that has blown both hot and cold during 2003/04.
They got off to a flying start when winger Ugo Monye sped in from half-way after only a minute of play, with Andy Dunn converting from the touchline.
Van Gisbergen slotted a penalty, but the visitors soon forged further ahead when flanker Pat Sanderson nipped in for a second try, which Dunne converted for a 14-3 lead.
But the sin-binning of Quins lock Karl Rudzki five minutes before the break saw the hosts use their extra man to get blindside George Skivington over the line for an unconverted try.
Although Quins centre Mel Deane almost grabbed a third score for the visitors, a Van Gisbergen try and conversion in first-half stoppage time saw Wasps go into the interval ahead 15-14.
Sanderson put the visitors back in front just 41 seconds with a converted try, but Wasps upped the gears and went about looking for the lead.
Harlequins absorbed the pressure and kept the champions at bay for a full 40 minutes, but – with just a minute to go – Quins prop Laurent Gomez was sin-binned.
As they had in the first half, Wasps used their numerical advantage wisely and the deadlock was finally broken by Erinle in injury-time and Harlequin hearts broke seconds later as Van Gisbergen stepped up to convert – grabbing almost all the points in the process.
For London Wasps:Tries: Skivington, Gisbergen, ErinleCons: Van Gisbergen 2Pen: Van Gisbergen
For NEC Harlequins:Tries: Sanderson 2, Monye Cons: Dunne 3