Nicholson looped the ball over Gills keeper Stuart Nelson from outside the box to reward the home side's second-half persistence.
Martin Allen's men had surged ahead when ace Dean Burton smashed in a 52nd-minute volley as United failed to clear their lines following Tom Flanagan's lofted ball down the left.
Playmaker Nathan Craig scored a superb 55th-minute leveller as the Gulls maintained their unbeaten home League Two record, curling in a pacey free kick from wide on the right and over Nelson.
The goal sparked a series of attacks from the home side and top scorer Rene Howe came closest to putting Torquay ahead with a superb shot on the spin that bounced off the outside of the post from 18 yards.
Nelson did well to flap an in-swinging corner from Craig off his line as the home side pressed for a goal. But he could do little about Nicholson's audacious winner. The defender buried his 85th-minute winner to maintain Torquay's impressive record.