The Fitbit trackers only count steps and use those as an indication of horizontal distance travelled. There is no facility to do anything with gradient since the system isn't set up to understand the concept or do anything useful with it.
Mike | London, UK
Blaze, Surge, Charge 2, Charge, Flex 2 - iPad Air 2, Nokia Lumia 925 (Deceased), iPhone 6
Take a look at the Fitbit help site for further assistance and information.