But it wasn't clear whether Gasly would be able to race at the upcoming United States Grand Prix, as before replacing Kvyat he had been competing in the Super Formula championship, the final round of which will take place in Suzuka, Japan, at the same time as the US GP, preventing him from attending both.
With one LMP1 World championship to his credit for the German powerhouse, another all but guaranteed once the current season ends, and an overall victory in June's 24 Hours of Le Mans behind the wheel of the all-conquering 919 Hybrid, the Kiwi has more than solved any lingering questions as to whether he's made from extraordinary stuff.
"I want to say a huge thanks to Red Bull and to Porsche", he told the Toro Rosso site.
With enough super licence points to race in F1, Hartley is now contracted to Porsche and competing in the World Endurance Championship, where he could win the title this weekend at Fuji, Japan.
"What an wonderful feeling!" said Hartley, whose F1 hopes looked to have ended when he left Red Bull in 2010. COTA is a track I really enjoy and one I have raced at recently.
The New Zealander will be the second driver to debut with the Italy-based team this season after Gasly was drafted in for the Malaysian and Japanese Grands Prix in place of under-performing Russian Daniil Kvyat.
It could be a busy two weeks for Hartley as he'll be racing with Porsche at Fuji in the World Endurance Championship, and competing for the category's title.
Gasly was initially handed Daniil Kvyat's F1 auto for two races, Malaysia and Japan, however, Carlos Sainz's early move to Renault means he'll be staying with the team for the remainder of this season.
And while it might seem likely that Kvyat will stay on, he has now been demoted twice by Red Bull - once from the senior team back to Toro Rosso in May 2016 and then dropped altogether after this year's Singapore event.
"With all the racing experience accumulated over the years, we are convinced that he will do a fantastic job for us".
Hartley, the first New Zealander to earn an F1 drive since Mike Thackwell in 1984, added: "What an wonderful feeling".