I gotta know gamepress..

Dragon Tail + Crunch Gyarados deals a ton of damage to electric types. Yes, it dies fast but, none of Zapdos/Raikou/Jolteon can finish a 3000cp Gyarados within just one cycle. Therefore, Gyarados can launch some Crunch to its worst enemies. Also if you switch in an Electric Type before the battle, you are gonna take the first two Dragon Tail from Gyarados for sure.

Gyarados is a bad defender by itself - but it forces switch Machamp. Putting a Gyarados after the Blissey/Chansey/Snorlax trio and then a Rhydon/Donphan thereafter is a very good gym setup.

Edit: Grammar

The problem with gyarados as an attacker is the lack of stab fast move. Give it a water or even flying fast move and it will be relevant again. I hope niantic will do that in the next move rebalance.

Well it's already been explained how it hits hard at it's counters as a defender if it's got DT + Crunch, but also as an attacker, specialists are more valued than before to counter specific raid bosses rather than having generalists that work against most raid bosses e.g. Dragonite.

Gyarados isn't that good of a specialist since it's only STAB move is Hydro Pump, which causes it to be outclassed by Vaporeon as a water specialist, and as a generalist it is also outclassed by Dragonite and Tyranitar in most situations, plus Mewtwo for those fortunate to get an early EX pass.

Nothing not named Blissey and MAYBE Slam Snorlax is a good defender by itself.

Gary does a decent job of putting out some damage on electric before it dies.

Gary is also a good switcher. Put it between than Snorlax/Blissey to force out the Champ or between a Steelix/Eggy to force out the Fire. Or Rhydon/Golem to force out the water.

However, Gary deserved his high rating BEFORE Raikou and to lesser extent Zapados. When the best electric was Jolteon, it hurt jolteon with DT/Crunch.

That said, when they redo the Defender list, I expect Gary to lose a full rating, or two. It's more a B-C defender now that stronger electrics are available to players.