The fitness guru, aged in his 40s, never could have imagined that the decision to open a single gym in 2013 in the Sydney suburb of Paddington would lead to such a lucrative enterprise.
Gilchrist and Rob Deutsch founded the company in 2013, which now has more than 1500 studios, with about 2800 franchises in 63 countries.
Deutsch is no longer involved in the company after leaving in February last year.
The brand is known for its high intensity interval training (HIIT) classes.
F45 said last week it was aiming to sell about 20.3 million shares priced between $US15 and $US17 apiece to raise up to $US345 million ($NZD492 million).
The brand has grown a lot in the last decade, which is in large part thanks to the interest it has attracted from celebrities.
Hollywood star Mark Wahlberg took a special interest in the company in 2018, after taking his first class and then successfully requesting for a minority stake in the company.
Wahlberg came to Sydney in December last year for his "Wahlburger" business but while he was there, he dropped by a Wooloomooloo F45 fitness studio for an early morning workout.
To promote F45's launch on Thursday, the American actor stood on the ground floor of the New York Stock Exchange building alongside Gilchrist and talked up the company.
"Die-hard fitness enthusiasts who don't have the schedule, got to do it in the middle of the night or first thing in the morning, don't want to get on a bike. That's fine. But eventually that becomes stagnant and boring," Wahlberg told local media.
"You want to be in there with the energy of people working out with you, alongside you, inspiring you, pushing you and supporting you.
"The energy (at F45) is absolutely incredible."
Another celebrity also jumped on board – famous music executive Steve Aoki.
The musician, valued at $US95 million ($NZ135 million), was spotted DJ-ing as F45 launched.
The company's move into the stock exchange comes months after terminating a merger.
F45 agreed in June last year to merge with Crescent Acquisition, a special purpose acquisition company, but later cancelled the deal as the Covid-19 pandemic shut several of its studios.
About 86 per cent of the company's total studios were open as of March 31, according to its filing.
