if what you care about is legendaries, gyarados is unfortunately going to be totally outclassed. the only ground-type legendary pokemon outside of groudon are landorus and zygarde. both of those are doubly-weak to ice, so gyarados will be a suboptimal matchup, and zygarde isn't weak to water.
that said, if primal groudon becomes a raid boss, it will be doubly weak to water, so gyarados may find use there for TDO.