The Lilywhites had trailed for much of the game after Udoyen Akpan put the visitors ahead from close-range in the 25th minute, but PNE showed plenty of promise in the second-half and were rewarded when Garstang curled into the top corner in the 94th minute.
The game started slowly, with both teams battling for possession in midfield, but the visitors created the first real chance when Callum Roberts anticipated an attempted chip by Akpan and saved well after 11 minutes.
Opportunities were sparse in the opening stages, although Alex Wood went close when he cut inside from the right and had his low strike saved at the near post.
The home side looked like they were starting to find some rhythm at that point as they kept possession well, particularly on the right flank, but they were pegged back when Tranmere took the lead in the 25th minute.
Tom Croughan’s attempted shot from the right proved to be an effective cross, as it gave Akpan a simple tap in to break the deadlock from only a few yards out.
PNE responded well, though, as Garstang forced a near post save from the opposition goalkeeper on the half hour before Barry hit the rebound over, and that pair nearly combined again minutes later but the former’s cross was collected.
Joe Brett shot straight at Roberts for Tranmere shortly before the break, while Wood curled over from the edge of the box at the other end.
Wood then found Garstang with a defence-cutting pass in what was the final chance of the half, but the attacker’s first touch was collected by the Tranmere ‘keeper.
Although the game continued to offer few scoring opportunities, PNE improved after the restart, with first-half introduction Ethan Walker proving a constant menace on the left wing.
He was found by Oscar O’Neil’s superb through ball with just over 20 minutes remaining but his cross went over. He almost combined with fellow substitute Myles Mason shortly after – acrobatically volleying towards goal from his teammate’s knockdown on the edge of the box.
Mason showed further promise in an advanced area in the 81st minute after he received Garstang’s excellently-weighted pass on the right side of the box, but his low strike trickled just wide of the far post.
Walker almost profited from a miskicked clearance by the Tranmere goalkeeper with two minutes to go, but his instinctive volley in the box went just wide.
The U15 winger then fired just over from a free-kick in added time in what was an encouraging display by the youngster, but his teammate Garstang went one better with his set-piece strike in the dying moments of the game.
With the ball placed just left of centre outside the box, Garstang stepped up and curled the free-kick right into the top right corner – earning PNE a point in outstanding fashion.
Academy manager Nick Harrison suggested the late equaliser was a just reward for a positive second-half display.
“I thought based on our second-half performance, we probably deserved to win the game,” he said.
“In the second-half we were very good; we dominated them and had a number of chances. I was only frustrated with our performance in the first-half and in the second-half I was more than pleased.
The second-half improvement came after some words of encouragement from Harrison at the break.
He added: “We were disappointing in the first-half because we didn’t win enough second balls, headers and tackles or do the basics too well. I felt we just weren’t at the races, even though we had quite a number of chances to score ourselves.
“We just told them that they needed to up their game. We said they needed to win the tackles and the headers, the second balls, be more positive and play on the front foot.
“We actually finished the last five minutes of the first-half in that fashion as well, so we just told them to take that into the second-half, which I’m more than pleased that they did.”
And the academy boss couldn’t ignore Garstang’s fantastic free-kick which earned his side a point.
He said: “The free-kick at the end is fantastic and just shows the ability that Harry Garstang has got. He’s shown great composure with only a couple of minutes to go to put it in the top corner and get a point for the team.
“We all know what Harry can do, he’s a very good player. I think he’s just one of a number of players who has got the ability to take those free-kicks; we had one just before it where Ethan had a chance.
“I was pleased that Harry stepped up for the second one because we know what he can do with the ball. There was a lot leading up to that with a couple of minutes to go really, but prior to that in the second-half we had some really good chances that we probably should have scored from as well.”
PNE line-up: Roberts, Creech, Burgoyne, Day, Campbell, Smart, O’Neil, Wood, Barry (Mason, 63), Davidson (Walker, 30), Garstang. Subs not used: Stead, Cottam, Baxter.