Nineteen-year-old England youth international forward Gelhardt was brought on as United's third and final substitute during Saturday's Premier League clash at Stamford Bridge at which point the Whites were 2-1 down.
Just 81 seconds later, Gehardt netted with his first touch of the game, providing a clinical finish to a fine Tyler Roberts cross in front of an ecstatic away end.
Yet joy turned to despair for the Whites deep in second half stoppage time as Mateusz Klich was penalised for a foul in the Blues area on Antonio Rudiger and Jorginho stepped up to convert his second penalty of the game.
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It meant a special day for Gelhardt ended in a 3-2 defeat but the Leeds teen quickly set his sights on his side righting the wrongs.
"Obviously it would have been nice to have got a point in the end," said Gelhardt, speaking to LUTV.
"It was a disappointing end and on another day we get a point out of it and we go home but I think the penalty was unlucky.
"The lads, obviously we hate losing but we have always got time to put it right."
Reflecting on the feeling of scoring his first Premier League goal and Whites head coach Bielsa's instructions in sending him on, Gelhardt said: "When I come on I always just try and impact the game and the manager just told me to get on it.
"When the cross came in my eyes lit up and then when it went in I almost didn't know what to do.
"I didn't know how to react but it was a special moment.
"It was an unbelievable experience scoring my first Premier League goal.
"I just want to thank my family and everyone who I have worked with coming up through at Wigan and then obviously joining here, all the staff, all the players who have helped me and especially my family.
"I am sad they couldn't be here but I know they were watching at home so I am glad.
"It was unfortunate at the end with the penalty but that's football."
Asked if he could sum up what was going through his head after scoring, Gelhardt smiled: "I can't remember what happened, I'd have to watch it back.
"But I know I was just like 'oh my god', I couldn't believe it and then seeing all the fans go crazy, it was just crazy."
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Thank you Laura Collins