In January, a GoPro support member ended up sending out a "Dear John" letter to Windows Phone users. Right away, with its talk of offering legacy support for Windows Phone, it was apparent that the wind had shifted. And on the GoPro website, Windows Phone was no longer listed as a location where one could pick up the GoPro app.
This isn't totally a Windows Phone problem. GoPro is facing competition and the company apparently doesn't have the wherewithal to continue supporting Windows Phone with the hope that it will eventually amount to something. In the meantime, the GoPro app is still available in the Windows Store.
GoPro app no longer supports Windows Phone and will not be built for Windows 10 Mobile
source: GoPro via MSPoweruser