Anyone thinks Outrage Gyarados is useful?

Useful if you don't have a good ice pokemon for attacking dragons.

It will be useful if a Dragon pokemon like salamence will become a raid boss one day. But it's a small niche. Not many of us will probably feature 6 powered up ice pokemon (for me lvl 37 it's two with Lapras and Cloyster). Most of us though will have Dragonites maxed who will deal huge damage but also die quickly. So in the end it depends on the size of your raidgroups if you can afford to throw in glass cannon like Dragonites or if groups are smaller have to go towards survivability. DT Lugia would be a good “tank“ if you like the tank philosophy but I think DT/OR Gyarados could be very useful as someone who will not faint with the first OR coming from the raid boss and deal a good amount of damage at the same time. I used mine as defender back in the past but didn't change the moves for that purpose since I don't use Gyarados at all atm.

to get this out of the way first - it's probably the worst defensive move, but if you have a gyrados on defence you are doing something horribly wrong.

as for attack, i've completely gone off gyrados now i have decent dragonites and ttars as it has no type advantages that aren't better covered by those 2 or vaporeons. conceivable that a double dragon one might be handy in gen3 but i'd not waste TM's on it

I used TM's on Gyarados back in July thinking "this is awesome, I can finally have useful moveset pairings since I never use Gyarados". 4 months later I still never use my Gyarados for anything. They are simply taking up space in my storage. At least now I have double dragon, double dark, and hopefully some day in the future double water movesets instead of awkward mismatches.

I've given this some thought too, as I just got a decent IV shiny karp.
DT/Outrage on Gyarados is somewhat useful against Dragons, but if you look at the matchup, Gyarados isn't weak to Dragon, but also doesn't get Stab. So if Dragonite has the same attack, D'Nite is doing 110 x 1.2 = 132, while Gary is doing 110 x 1.4 = 154 (before Att/Def mods). Small advantage, equivalent to having STAB vs. not having STAB. A much less efficient approach than using Ice against Dragonite, doing 1.96 x damage.
Or you could argue that DT/Outrage is a strong general attack that's good against most things. Sure, but if that's the case, why not use it coming from a Dragonite to get STAB? If you want to do general damage against ice, rock, fairy, or dragon, use something different (like fire/rock, water/plant, steel/poison, or ice respectively).

Gary with DT/O? I don't see it as being overly useful. Maybe someone else out there can educate me.

Personally, I'm waiting for after the Gen3 update to evolve my shiny. Maybe Gary will get a water quick move, making him more relevant - right now among players with a large arsenal of mon, Gary is lightly used at best - Vaporeon is still the best water attacker (and probably will be until Kyogre, whenever that happens).