Gyarados's special attack base is crap, so Surf / Aqua Tail isn't recommanded. The first two moves are correct, Ground and Ice give it great coverage. Add Dragon Dance for a boost in attack and speed, and Waterfall for STAB. Razor Claw isn't your most useful item, so either Leftovers or Life Orb would work better. While Hasty nature raises its speed, it decreases defence, which I don't think Gyarados would be very fond of, Adamant nature instead.
Why 4 Grass-type moves on Leafeon?
Leafeon @ Leftovers
Jolly Nature
EVs: 124 HP / 172 Atk / 212 Spd
- Swords Dance
- Baton Pass
- Roar
- Leaf Blade
On top of its supporting abilities (Roar and Baton Pass), Leafeon already has a great attack base, and with the use of Swords Dance included, it has the advantage of using its powerful STAB move, Leaf Blade.
Blissey surely can't do any damage with that pitiful 10 attack base, so Earthquake is not an option. Stealth Rock does give you support, but Blissey generally has other more useful moves to use.