Fully EV'ed, IV'ed Modest Milotic has a very, very slim hope of OHKOing neutral natured fully IV'ed and un-EV'ed Gyarados with full powered HP Electric. It's a guaranteed OHKO with Magnet.
On the other hand, fully EV'ed, IV'ed Adamant Banded Gyarados has no chance of OHKOing neutral natured, fully IV'ed and un-EV'ed Milotic with Thunder Fang. It needs SR to OHKO, and even then, it's not guaranteed. The only guaranteed OHKO is a CH.
It depends on the speed tie.
If Gyara is facing a Magnet Milo that goes first, it's dead. End of discussion.
If LO Gyara managed to DD first in the face of a Milo with no power boosts, Gyara is very likely to win.
Then again, if SR is there to turn the tables...