This year's Moto2 champion and runner-up respectively will ride under contract to the Austrian marque with its satellite team when they step up to the premier class next year.
Both have already ridden the RC16 which KTM fields in MotoGP, at Misano in September, but their 2022 campaign begins in earnest this evening (AEDT) with the post-season test at Jerez.
Gardner wrapped up the intermediate class title by finishing 10th last weekend at Valencia, where Fernandez broke Marc Marquez's 10-year-old record for rookie season Moto2 race wins by taking his eighth victory of the campaign.
Gardner will continue to ride Bike #87, a reference to father Wayne's 1987 500cc world championship title, and Fernandez #25.
The Jerez test runs for two days.