Hessenthaler continues to belie his 37 years, and was outstanding as first division Gillingham survived a spirited display by third division Torquay.
Torquay had made and missed several good chances before Hessenthaler struck in the 69th minute. In the first half Gills keeper Jason Brown only just saved with his legs from Torquay's Scottish striker David Graham.
And in the 34th minute Graham controlled a Kevin Hill cross from the left and volleyed wide when he should have hit the target.
In an entertaining tie full of slick passing and end to end movement, Guy Ipoua was always a threat for Gillingham. But when he missed a wonderful chance in the 49th minute, it looked as if the Kent side might be on the end of a shock defeat.
Martin Gritton, Alex Russell and Hill all went close as Torquay made their bid for victory midway through the second half. But Hessenthaler settled it in the 69th minute.
Mark Patterson crossed from the right and Kevin Dearden blocked Kevin James' shot from Ipoua's knock-down, but the Gills boss pounced to bury the rebound from ten yards.
"I thought we did a very professional and patient job," said Hessenthaler, who took himself off with ten minute to go. "After the last few results, we could easily have come here with our heads down but we didn't."
Torquay boss Leroy Rosenior said: "When you play quality sides and you get a chance, you've got to take it. And we didn't.
"But I thought we deserved something from the game, and I was delighted with the way we played. It was the performance I was looking for above anything else, and our young side can only learn from games like this."