What do you think?

I use a non-powered up level 20 93% IV B/HP Gyarados as an attacker all the time and it's quite effective.

I'd power it up if you are in need of a higher CP thing to secure a place in the middle of a gym.

Attacker against what?
My problem with gyarados (and other "generalist") is, there's really no matchup where they truly shine and I'd use them instead of other that pokemon would get the job done faster/better because of type advantages.

Gyarados is a strong attacker regardless of the moveset, as it has very high stats, but doesn't really have a place other than as a high CP "defender" maybe you could use him to take down snorlax' and blisseys, but again... there are other mons that can do that job better.

10. B/HP and DT/HP are both very good. best is to try them both and go with the one you like best.