It’s not, unfortunately, and that sucks. That’s why the Gadgetbridge compatibility list specifically warns about that whenever relevant. (As far as Gadgetbridge not being present on iOS, sorry, you knew what you were buying when you came into the Apple store.)

It is, however, a very mild case of cloud suckage, because the only thing the vendor learns in the process is an association between an email address and a device, after which you (delete the vendor app and) never communicate with them again. (You could in principle use a temporary email address if you’re particularly adventurous and don’t plan to resell.)