The Hornets were deadlocked until the 73rd minute mark when Purewal converted from point-blank range.
Daniel Moore made two changes to his side as Liam Murray and Olly Martin boosted Hebburn’s attacking options.
Murray nearly created the opener as the midfielder superbly lofted a ball through for Joe Walton, who hooked over inside the area.
Tom Devitt then slashed a shot off target after Robbie Spence crossed to the defender at the back post.
At the other end, Nathan Tyson found Matt Tootle at the back stick but the experienced right-back couldn’t take advantage on the counter.
Martin fired a free-kick straight at the wall on 33 minutes, and the attacker later slashed a strike over from the edge of the box.
Tyson had Grantham’s best chance straight after half-time, as the former Nottingham Forest man cut inside before stroking an effort off the crossbar.
Martin nearly found the net with 49 on the clock as he got on the end of Shaun Newbrook’s long kick, however the Hebburn attacker couldn’t keep his low shot on target.
Moments later, Murray proved a handful for the Grantham defence and drove an opportunity wide of the post from range.
The duo of Murray and Martin nearly broke the deadlock as the pair linked up before the former dipped a shot wide after a slight deflection.
Newbrook played a vital role in keeping it level on 57 minutes as he blocked Daniel Cocks’ shot one-on-one.
The Gingerbreads again went forward, with Bradley Munns volleying Joe Stacey’s cross past the post from the edge.
Purewal got the all-important goal midway through the second period when Jack Donaghy’s long throw was flicked on by Robbie Spence, before the forward stabbed home at close-range.
It could’ve been two when Ashton Hall’s pass didn’t reach the goalkeeper and Liam Henderson intercepted, but the substitute hit wide in an excellent position.
Hebburn defended well in the dying stages, and Henderson even threatened with an effort from distance, as Moore’s side held on to the three points.
Grantham Town: Haystead, Tootle (Munns 45’), Stacey, Waldram, Wright, Hall, Shaw, Dunn, Lee, Cocks, Tyson
Unused subs: Gadomski, Jemson, Hayes, Tait
Bookings: Hall (32’)
Hebburn Town: Newbrook, Devitt, Donaghy, Reid, Phillips (Henderson 61’), Spence, Thompson, Murray, Martin (Harrison 82’), Walton, Purewal
Unused subs: Coulson, Simpson, Moore
Goals: Purewal (73’)
Bookings: Henderson (81’), Murray (88’), Thompson (89’)
MOTM: Sean Reid
Referee: Chris Everald
Attendance: 286