The head-banging foursome gave a unforgettable performance on the balconies of the historic Conciergerie palace along the banks of the Seine on Friday night with a song evoking the guillotine executions of the French Revolution.
They were joined by opera singer Marina Viotti, for "Ah! Ca ira" ("Ah! It'll be fine"), based on the famous revolutionary song of the 1790s.
Viotti was still riding high from her appearance in front of more than a billion TV viewers when AFP spoke to her on Monday.
"It's dizzying," the 38-year-old French-Swiss mezzo-soprano said.
Viotti has dates coming up at Milan's La Scala and the Paris Opera, but she is no stranger to metal, having performed with groups Lost Legacy and Soulmaker.
She was overjoyed to bring the music to a wider audience.
"I've read comments on social networks saying 'I never listen to metal but, this one, it's great, it gave such energy to the show,'" she said.
She hopes it will help change the image of metal and finally rid the genre of its "Satanist" or "violent" cliches.
A surprise inclusion in the Olympics show, Gojira are a French band that have won over metalheads around the world with their pulverising guitars and earth-shattering drums.
"It's crazy, a very nice surprise and a world first for metal," said Corentin Charbonnier, a doctor in anthropology and a French researcher on metal music.
"Right now, we're living in a golden age for metal," he said.
Charbonnier helped curate France's largest-ever exhibition about the genre at -- of all places -- the Paris Philharmonic, which is running until September 29.
Metal is usually traced back to English group Black Sabbath in the early 1970s, merging with glam in the form of Kiss around a decade later and finding its textbook form with US band Metallica in the 1980s and 1990s.
It is now a fully established genre even in France, with a dedicated festival -- Hellfest -- that attracted around 240,000 fans to its latest edition last month.
Metallica headlined the festival for the second time in three years.
"What is interesting in the current revival of metal culture is that some want to be recognised and others want to stay in the shadows, in the counter-culture," Charbonnier said.
The Olympics show has perhaps made it harder to stay niche -- Gojira saw their Spotify streaming numbers jump by 282 percent in France over the weekend, and 129 percent worldwide.
"Some people worry that we risk losing the essence of the music" if it grows too mainstream, Viotti said.
"But, in my eyes, we must move in the direction of union, gathering, sharing, building bridges."
