Experience with Magneton?
How does Magneton fare against the new Gyarados? In old times, Magneton was clearly inferior to Jolteon in the one important task for Electrics, beating up Gyarados. But with the move change, both sides of the equation changed:
Gyarados has now Dragon Tail, which is not efficient against Magneton. This nearly balances the higher STA and DEF of Jolteon.
Magnetons legacy move Discharge is now 3*65, for a total of 195 damage; this is on par with the strongest moves of Jolteon. Magnetons new Zap Canon deals 140 damage in a single attack, much better than Thunder and nearly as much total damage as Jolteons two-bar Thunderbolt.
Answers
Normally Gyarados will kill the Magneton just by spamming Dragon Tails, and then the Hydro Pump kills the next Pokémon in your lineup. Bite and Dragon Breath are trash on defense, even before the update.
Against Dragonites, though, truly shines against SW and DB ones, things will nearly become the same if the Dragonite has DT.