The Red Bull-owned team said the switch "gives us an opportunity to make a more informed decision regarding our 2018 driver choices".
"This is not a case of goodbye for our Daniil, as he still remains part of the Red Bull Family," the team added in a statement, although the future does not look bright for the Russian.
Gasly, the 2016 GP2 champion, is the leading contender to replace Spaniard Carlos Sainz, who will be leaving Toro Rosso on loan to Renault next season.
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Pierre Gasly of France and Red Bull Racing talks to engineers in the garage during day two of F1 in season testing at Hungaroring on August 2, 2017 in Budapest, Hungary.

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Kvyat could be retained but has scored only four points in 14 races compared to Sainz's 48, with the team locked in a tight midfield battle.
They are sixth overall, seven points behind Williams and 10 ahead of Renault.
Renault had hoped to secure Sainz before the end of the season but Tuesday's move indicates the departing Jolyon Palmer will now see out 2017 in that seat.
Toro Rosso, who are switching from Renault to Honda engines next year, can ill-afford to lose Sainz with six races remaining.
Gasly is already under contract to Red Bull who use Toro Rosso as a feeder outfit for their main factory team. Japan follows immediately after Malaysia and is familiar territory for the 21-year-old.

Pierre Gasly of France and Red Bull Racing drives in the Red Bull RB13 TAG Heuer on track during day two of F1 in season testing at Hungaroring on August 2, 2017 in Budapest, Hungary.

Image credit: Getty Images

He may miss the U.S. Grand Prix on Oct. 22, however, since that clashes with the final round of the Super Formula championship at Suzuka.
"He (Gasly) is the next in line at Red Bull for this opportunity and he has shown that he deserves it," team boss Franz Tost said.
Kvyat has already been through the process, moving up to Red Bull Racing in 2015 and suffering a demotion last year when he swapped places with Dutch teenager Max Verstappen.
"For a variety of reasons, some of them due to technical problems, but others being mistakes of his own making, Daniil Kvyat has not really shown his true potential so far this year," Tost said.
"This will give us the opportunity to evaluate Pierre on track during a proper race weekend."
Gasly's arrival means France will have three drivers in Sunday's race at Sepang, the newcomer joining compatriots Romain Grosjean (Haas) and Esteban Ocon (Force India).
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