The home side, needing at least two goals on the night following a 2-0 first leg defeat, dominated for an hour but were denied a breakthrough time and time again by the heroics of Town keeper Scott Brown.
Jermaine McGlashan's goal 15 minutes from time appeared to put the tie beyond doubt, but substitute Taiwo Atieno headed home a corner on 85 minutes to give Martin Ling's side reason to hope for a miracle finish.
Marlon Pack made absolute certain two minutes later, however, with a blockbuster free-kick as Mark Yates's men completed the double over their play-off opponents.
Cheltenham and Crewe Alexandra - who made the final the previous day with a 3-2 aggregate victory over Southend United - will now do battle for a place in League One at Wembley on Sunday May 27.
Torquay, who were defeated by Stevenage at Old Trafford a year ago, wasted little time searching for the goals they desperately needed to stand a chance of making a second straight play-off final and Brown showed great reflexes tipping Eunan O'Kane's effort over on 10 minutes.
The attacking midfielder then had a low cross helped on by Danny Stevens six minutes later, with only Keith Lowe's intervention preventing Ryan Jarvis from tapping home at the far post.
Jarvis had a chance pushed around the post by Brown on 18 while a corner caused chaos for the Robins' defence after 25, the ball pinballing off a red shirt and onto the upright.
Just 60 seconds later, Brown was called into action again to keep out Stevens's header while Kevin Nicholson's free-kick on the stroke of half-time came back off the crossbar with the goalkeeper left stranded.
Kaid Mohamed served as the away side's biggest threat on the counter, and his best chance came early in the second half when he forced a good save from Robert Olejnik in the United goal.
Lee Mansell's header on 53 minutes was then deflected dangerously towards goal by defender Alan Bennett, forcing Brown to acrobatically tip over the bar once more to prevent an own goal.
Mohamed came close again on 67 minutes after fine work from Sido Jombarti as the away side finally began to grow confident with time running out for Torquay.
And eight minutes later his through ball teed up McGlashan's finish across Olejnik from a tight angle, the midfielder's second goal in the tie, which appeared to confirm Cheltenham's place in the final.
Atieno's clever guided header levelled the score on the night 10 minutes later, finally giving Torquay a deserved goal in the semi-final, but what little hope they had of finding another two in five minutes was snuffed out 120 seconds later.
Brian Saah cynically brought down Jeff Goulding to prevent the substitute going through on goal, and though the defender was lucky to escape a booking it would not matter as Pack's strike from the free-kick soared beyond Olejnik and in.
It was a cracking finish to a great performance over two legs from Town, who are now 90 minutes away from promotion to the third tier.