Gyarados is my 2nd favorite Pokemon from Gen 1 (behind Dragonite) and my 2nd favorite water Pokemon of all time (behind Greninja). I will almost always have him on my team for a playthrough of any Pokemon game. Here are some really fun and competitive sets I have used throughout the years that are still viable. Disclaimer, I have a strong aversion to full glass cannon Gyarados. So if that's what you're expecting then keep scrolling!
Specially Defensive Physical Sweeper:
Gyarados @ Assault Vest
Ability: Moxie
Tera Type: Flying -OR- Ground
EVs: 200 HP / 56 Atk / 252 Spe
Jolly Nature
- Crunch (Psychic/Ghost Coverage)
- Ice Fang (Grass/Flying/Dragon Coverage)
- Earthquake (Electric/Poison/Steel coverage) / Tera Blast (Grass/Fighting/Bug Coverage)
- Waterfall (STAB)
This set takes advantage of Gyarados' high sp def! Ground will always be the preferred Tera type for a Gyarados sweeper but flying can work too. Tera Blast flying has some overlapping coverage with Ice Fang but it's fine since Tera Blast can hit other Water types as well as ice types where as Ice Fang cannot. Earthquake can only be used in singles unless you pair it with a flying Pokemon or one that has protect. Tera Blast can be used in singles or doubles. Tera Ground can also be used with Tera Blast in doubles and offers the same coverage as Earthquake without dealing damage to your ally. Waterfall is the standard water STAB but if for some weird reason you prefer aqua tail then go for it.
Specially Defensive Physical Sweeper #2:
Gyarados @ Loaded Dice
Ability: Moxie
Tera Type: Dragon -OR- Steel
EVs: 196 HP / 56 Atk / 248 SpD / 8 Spe
Careful Nature
- Dragon Dance
- Earthquake (Singles only) / Iron Head (Singles/Doubles)
- Scale Shot
- Waterfall
This set takes advantage of Gyarados' high sp def as well but without the assault vest. I put Steel as a second Tera option but that kind of defeats the purpose of taking Loaded Dice + Scale Shot. It could definitely still work just not as optimal. The EV spread looks weird but the purpose is to be able to tank a hit or 2 so you can set up with dragon dance (attack and speed boost) and then sweep with moxie (attack boost on KO) and scale shot (buffed by loaded dice and Tera Dragon AND you get a speed boost after every use!). Earthquake is the standard coverage move but since you are Tera dragon then you need a way to remove fairy threats immediately, so that's why I prefer to run Iron Head with this set. If you do choose to run Tera Steel for more bulk then Iron Head becomes STAB. If you're using this set for doubles and you have an aversion to using Iron Head for whatever reason then just take Ice Fang or Protect or Taunt.
Bulky Physical Phazer/Hazard Support:
Gyarados @ Leftovers -OR- Rocky Helmet
Ability: Intimidate
Tera Type: Normal -OR- Water
EVs: 240 HP / 172 Def / 96 SpD
Impish Nature
- Body Slam (Tera Normal STAB)
- Dragon Tail (Phase)
- Waterfall (STAB)
- Thunder Wave/ Dragon Cheer/ Helping Hand/ Rain Dance
This is a niche but very flexible set that requires specific Pokémon (hazard set-up and stall) for it to work. Lum Berry is the standard defensive item for a sweeper set but this is a more bulky set so the sustain from Leftovers is better. Rocky Helmet also works and synergizes well with the overall build and what this set is trying to accomplish. Intimidate is OP and I don't think it needs much explanation. The EV spread and impish Nature is for bulk. This is NOT a sweeper set. This is a Bulky Phazer set that is used to support hazard teams. Dragon Tail is your phase move to force your enemies to get poisoned/ take chip damage. The last move is completely dependent on whatever your team needs. Thunder Wave is standard and pretty OP but any of the other boosting moves is fine if that's what your team needs.