The Gills set a new club record for wins away from home in a Football League season by securing their 11th away win of the campaign at the Proact Stadium.
After regaining top spot in League Two with a 2-1 home victory over Dagenham & Redbridge last weekend, Martin Allen retained the bulk of his starting XI, Steven Gregory the only change, replacing the injured Andy Frampton in midfield.
After soaking up the Spireites early enthusiasm, Gillingham almost caught the home side cold with their first real sight of goal. Myles Weston’s eighth minute corner was flicked on by the head of skipper Adam Barrett but just wouldn’t sit right for Charlie Lee to slot home at the back post.
The visitors looked particularly dangerous from set-pieces and moments later another Weston corner saw Tommy Lee make the games’ first save, diving low to his right to turn away a Chris Whelpdale snap-shot from just inside the area.
For the home side Jay O-Shea rolled an angled shot across goal and wide of Nelson’s right hand up right, while Cody McDonald was sent one on one with the keeper by Steven Gregory’s 17th minute ball over the top but a deflection meant the Gills could only claim a corner.
The lively O’Shea was involved again for the Spireites on 23 minutes, making space to float a neat ball into the box from the left touchline, it was nodded into the path of Marc Richards but his volley from the edge of the D cleared the crossbar without testing Stuart Nelson.
The Gills were forced to reshuffle on 28 minutes as Matt Fish left the field due to injury. Deon Burton was his replacement, allowing Myles Weston to drop into the midfield and Chris Whelpdale to occupy the right-back berth.
It made little difference to the Gills momentum though with Joe Martin going close soon after with a low effort to the keeper’s near post from the left edge of the box, while Tommy Lee should sharp reflexes again on the half hour, racing off his line to thwart Charlie Lee’s long pass for McDonald.
Three minutes before the interval the home side spurned a great chance after O’Shea had helped a long clearance into the path of Drew Talbot, the Chesterfield midfielder raced into the box and attempted to curl an effort into Nelson’s top corner, ultimately his finish was well wide off the mark.
The Gills looked full of purpose after the restart and sure enough took the lead in the 50th minute. Myles Weston swung over a testing corner from the right and Cody McDonald rose above the home defence to head home the fourth goal of his second spell with the club.
The goal prompted a double substitution from the Spireites, including veteran striker Jack Lester, and saw the home side test the Gills twice in quick succession, first Whelpdale was perfectly placed to clear Alex Henshall’s header off the line before Conor Townsend fired just over.
Martin Allen acted quickly to check their momentum, introducing Callum Davies at centre midfield to protect the back four. Goalscorer McDonald was the man to make way as Weston and Burton formed a new strike pairing.
The switch put the visitors firmly back in control and Tommy Lee was called in to deny Charlie Lee’s goalbound header, midway through the half, before Deon Burton was unfortunate with a well drilled free-kick that scudded off the wall for a corner.
On 72 minutes Leon Legge did superbly well to block a Jack Lester shot from the left side of goal after Deon Burton had been left floored by a clash of heads, while soon after Nelson was well placed to cut out the danger from Henshall’s centre after Brindley had found him in space on the left.
Leon Legge came close to doubling the Gills advantage with a precision header from Weston’s 77th minute corner that was blocked at close range by Tommy Lee. Within moments Trotman had to block Burton from similar range after Legge had played Whelpdale’s free-kick to his feet.
Ten minutes from time Legge and Nelson combined to keep out Lester as he found space in the Gills area, while the same player headed over the bar moments later before Stuart Nelson was harshly booked for taking too long over a goal-kick.
Jack Lester did manage to find the net in the 90th minute but the linesman flag had already gone up for offside before the fourth official raise his board to herald a staggering seven minutes of added time.
In the fifth minute of injury time Anton Robinson went agonisingly close to wrapping up the points after a quick counter attack but he hooked effort from the left of goal dropped just the wrong side of the post.
There was still time for Myles Weston to find his way into the referee's book following a tangle with Drew Talbot before the official's final whistle sparked wild celebrations for the Gills travelling faithful.
Gillingham: Stuart Nelson, Matt Fish (Deon Burton 28), Adam Barrett, Leon Legge, Joe Martin, Steven Gregory, Anton Robinson, Charlie Lee, Chris Whelpdale, Cody McDonald (Callum Davies 59), Myles Weston
Subs not used: Tommy Forecast, Michael Richardson, Bradley Dack, Ryan Williams, Adam Birchall.
Chesterfield: Tommy Lee, Drew Talbot, Neal Trotman (Jordan Clark 80), Liam Cooper, Nathan Smith, Richard Brindley, Sam Togwell, Craig Clay (Jack Lester 53), Conor Townsend, Jay O'Shea (Alex Henshall 53), Marc Richards.
Subs not used: Josh Barrington, Mark Randall, Danny Whitaker, Jacob Hazel