Just a week after they combined to win the Daytona 24 Hours, Engel and Gounon became bitter rivals in the closing stages of the Bathurst race.
Engel and his #999 GruppeM Mercedes team-mates had gone into the final stages of the race in control, only for an in-car modem, provided by the event itself, to fail.
Replacing the unit meant a longer final stop which opened the door for Gounon in the SunEnergy1 car to sneak through to the lead.
Engel's desperate bid to get back in front resulted in the two factory drivers colliding at The Chase with 45 minutes to go.
The resulting drivethrough penalty for Engel left Gounon in the lead, the Frenchman then keeping clear of a late charge from Porsche's Matt Campbell and a recovering Engel to take a dramatic victory.
Gounon and Engel were show on the broadcast embracing at the conclusion of the race.
The win was a second in a row for SunEnergy1 Racing and Kenny Habul and Luca Stolz, and a third consecutive triumph for Gounon, who also won in 2020 as a Bentley driver.
"It's unbelievable," said car owner Habul, who was able to clear his laps early thanks to entering the car in the Pro class.
"Luca and Jules just drove the balls off this car and I'm so proud of them. Thanks to Mercedes AMG. This is the greatest, I wanted to win outright. I drove well, I was proud of myself. I'm proud of everyone, proud of the team."
Campbell, Mathieu Jaminet and Thomas Preining finished second in the Manthey EMA Porsche while Engel, Raffaele Marciello and Mikael Grenier rounded out the podium.
The final quarter of the race started with the Marciello holding a comfortable 25 second lead over Jaminet in the #912 Manthey EMA Porsche.
The off-sequence #75 SunEnergy1 Racing Mercedes, meanwhile, sat patiently around 45s behind the leader with Stolz at the wheel.
Right on the 9h25m mark one of the podium contenders was dealt a blow when the #46 WRT BMW was forced into an early stop.
The issue was carbon build-up on the rear of the car, the crew told by officials that it had to clear the mess so the taillights were visible, which had to be done in the garage.
10 minutes later the rest of the lead group started to make their stops, Sheldon van der Linde taking over the #32 WRT BMW for a double stint to get home while Campbell jumped into the #912 Porsche.
The leading #999 Mercedes then took its penultimate surface at 9h43m, Marciello staying in the car as Stolz and the #75 Mercedes moved back into the lead.
That left the #75 with a 35s margin over the #999, with another 30s back to Campbell in third and 15s more to van der Linde in fourth.
With just over two hours to go Stolz surrendered the lead when he handed over to Gounon for a double stint to the flag. That dropped him to third behind Campbell with a 42s margin to new leader Marciello.
There was little in the way of change across much of the next stint, Gounon able to make small gains on Marciello and Campbell, but not enough to become a short-term threat.
It wasn't until the last 90 minutes that Gounon started to make proper headway, closing to within 3s of Campbell as the gap to Marciello came down to just over 30 seconds.
At the same time GruppeM was dealing with some off-track drama, thanks to a failing event data logger that race officials told the team had to be replaced.
Negotiations between Supercars technical staff and GruppeM personnel over the replacement of an in-car modem continued for some time, the team ultimately forced to change the unit during the car's final scheduled stop.
That stop happened with 70 minutes to go, the longer service helping Campbell, who stopped for fuel and tyres on the same lap, to turn his 30s deficit into a 10s deficit.
Gounon then responded by taking his last stop a lap later, SunEnergy1 Racing opting against new rubber to ensure the Frenchman resumed ahead of Engel.
It was a fine margin, though, Engel crawling up onto the rear wing of the SunEnergy1 car within a couple of laps. The pair then staged a thrilling battle at the front, Engel desperate to use this tyre advantage to get past.
But Gounon's defence was impeccable as he kept the #999 Mercedes at bay lap after lap.
With a little under 50 minutes to go the battle reached boiling point when the pair almost made contact at The Chase. The following lap it reached an inevitable outcome when Engel tried to surprise Gounon at the same corner, but instead tipped the leading Mercedes into a spin.
Both cars got going again, Engel left with a 7s lead over Gounon. It was short-lived, though, with race control wasting little time handing out a drivethrough penalty to Engel.
The penalty handed the lead back to Gounon bu he was still within striking distance for both Campbell, and a fast-recovering Engel. It made for a tense run to the flag, Gounon holding his nerve to secure the victory by just nine-tenths over Campbell, with Engel another half a second back.
The #32 WRT BMW finished fourth ahead of the #888 Triple Eight Mercedes and the #46 WRT BMW.
The Pro-Am class, meanwhile, went to the #65 Melbourne Performance Centre Audi of Chaz Mostert, Fraser Ross and Liam Talbot, which also finished seventh outright.
The Silver class was dominated by the #10 International Motorsports Audi of Andrew Fawcet, Dylan O'Keeffe and Daniel Gaunt.
