Hartley has been tipped to take the seat at next week's US Grand Prix in Austin with the Toro Rosso team but is also rumoured to be in the running for a fulltime drive with the Red Bull-owned team next year.
The young Frenchman made his debut in Malaysia and competed in last weekend's Japanese Grand Prix alongside Spaniard Carlos Sainz, with Russian Daniil Kvyat dropped for those races. Meanwhile, Renault was pushing hard to drive Jolyon Palmer out and get Sainz in, who could potentially bring the team valuable points in the closing stages of the championship.
Gasly can win that title for Team Mugen who are powered by Honda, Toro Rosso's engine partners next year.
With that in mind, Toro Rosso and Red Bull have lined up Buemi, who raced for Red Bull's junior team between 2009 and 2011, to step in if a deal for Hartley can not be agreed.
A Toro Rosso spokeswoman was unable to clarify the situation on Thursday and did not know who would be driving in Austin.
One might wonder why Toro Rosso didn't simply hold on to Sainz for just one more round, which would have allowed Gasly to compete in the Super Formula finale and eliminated the need to find a fourth available driver.
Honda, who will provide the engine power for the Faenza based squad in 2018, had also stated they would not stop Gasly from racing in the USA, but would be happy for him to return to Japan if a replacement Formula 1 driver could be sought.
Hartley, 27, is a former Red Bull reserve driver who also tested for Toro Rosso in 2009 but has never raced in Formula One and has not been in an F1 auto since 2012, before the new generation of turbo hybrid engines.
However, acquiring the Kiwi will require some ironing out by the Italian Formula 1 team - the 27-year-old is now contracted to Porsche in the World Endurance Championship - a competition he could win this weekend with a solid performance in Fuji.