I have used thunder wave in gyarados before, but only in defensive sets. I good gyarados lead set that I created for gen 4 was this one:
Gyarados @ Lum Berry
Ability: Intimidate
EVs: 124 HP / 252 Def / 132 Spe
Impish Nature
- Thunder Wave
- Roar
- Waterfall
- Taunt
Well, yes... thunder wave is kind pointless in a sweeper gyarados since is a waste of a turn. Its much better a substitute than a thunder wave, especially since thunder wave got nerfed in gen 6.
In this case I guess that the usual "dragon dance" set is okay enough. You might wanna put his tera type ground to counter eletric types. Set his nature to jolly and make his ability "moxie" will probably be the better option. In the case of item I guess that sitrus berry is good enough.