One of my few 100% mons is a Gary with double dragon moveset. I know dtail is a legacy move now, but is it worth keeping it around when a much more useful option can be obtained? WWYD?
Similar dilemma to you... Instead I have dragon pulse on my 100 which is even rarer but even more useless than outrage.
But I'd say ask yourself, are you on the cusp of gaining 1 extra damage ball if you TM this gyarados? For me it doesn't matter, I can have 6 maxed WFHP gyarados and still not get an extra ball because 20 players pile into the raid. So I'm still keeping my legacy 100 as it is
Gyarados has such low dragon-type DPS due to no STAB that it's not really worth keeping for countering dragon-type raid bosses. Against dragon-type moves Gardevoir is now a much better option and might be replaced with an even better one in Togekiss later. Most dragons also have additional weaknesses thanks to dual typing that gives them other much more effective alternate counters than DT/O Gary. This is the case for all dragon-types in gen 1-3.
Keep it if you like keeping legacy mon, don't keep it based on its ability to counter dragon-type mons.
