The man who kept him out last time was French great Tony Estanguet who went on to win a third C1 gold in London.
This time it was the now-retired Estanguet, who was cheering on his countryman before hanging the gold medal around his neck.
While others wrestled with their craft on the choppy 242m course, Gargaud Chanut negotiated the 24 gates on the descent with silky precision, avoiding any of the red and green poles suspended above the foaming water.
Slovakia's Matej Benus continued his country's proud tradition in canoe slalom to take the silver medal.
Benus qualified for the team ahead of twice Olympic champion Michal Martikan and looked on for victory with a powerful final run, but Gargaud Chanut went 0.85 seconds faster.
Japan's Takuya Haneda was third, 3.27 seconds slower, to become Asia's first Olympic medal winner in canoe slalom.
Marseille's Gargaud Chanut is the first man to win the C1 title other than Martikan or Estanguet since 1992.
"This has been worth the wait, now I'm smiling but four years ago it was quite difficult," the 29-year-old told reporters at the pristine Rio whitewater park that locals could use as a place to cool off in the build-up to the Games.
"A lot of people have spoken to me about the pressure of trying to follow Tony, but I felt more support than pressure.
"London was a big disappointment to me because I was number one in the rankings and world champion. I've had four years quite complicated, but it went fast."
Japan has no pedigree in the sport but Haneda showed the benefits of moving to Slovakia to learn from the best when he was 18 -- leaving many medal tips in his wake.
Florence, C1 runner-up in Beijing and again in the C2 in London, was one of them.
"I was a bit tight on the first sequence and had a big time loss and it was unrecoverable from there," he said after clocking 15 seconds slower than the winner.
"You can't afford to make mistakes like that."