I've created an app for Gear Fit2 and got the following error when tried uploading it to Samsung App Store:

I've contacted Samsung support asking about the error above, and they basically said that only watch face development is supported for this platform for general public, however they do open the full dev capabilities for their partners.

UPDATE
Samsung has currently added support for GearFit2 and GearFit2 Pro, and you can now upload app binaries, not only watch faces to Galaxy App Store. You can also customize the emulator to make sure that it looks like a real GearFit2 watch as I've described here:
Emulator for Samsung Gear Fit 2 Tizen