I recently evolved a double dark Gyarados and turns out I really like it. It's great on a team in Alakazam and Gengar raids, especially because it can tank Focus Blast better than Ttar.
I assume by DT/H Dragonite, you mean Hurricane. If so, I love mine. Great to use in Machamp raids and as a generalist.
I would presume a double dragon Gyarados will be useful in the hypothetical future Dragonite raids. I have one but have not had a need to use it yet.