Legacy Gyarados usable?

I've faced a few (with twister) in gyms and they are actually slightly less annoying now - although I dodge a lot, so for the spammers (if there are any left) then they will still be annoying.

For attacking, its still not as useful as the HP/DP/Outrage versions.

Don't power it up.
Dragon Breath may have a little higher dps compared to Bite, but generates less energy.
Twister may be better than before, because generally charge moves got buffed, but Garados has MUCH better charge moves now, for offense and defense.

Even for attacking Dragonit, Bite or Dragon Tail + Outrage is way better.

With move slowdown, charge moves are much more important now. Dragon Breath/Twister might be marginally stronger in gyms, but is probably unfortunately still pretty sad for an attacker.