Gyarados is better than Slowbro for a several reasons:
- Gyarados has amazing stats; great attack, decent speed, and solid defenses
- Intimidate can be pretty helpful against common physical attackers you will face in-game
- Despite having a pretty shallow movepool, Gyarados gets Dragon Rage at Level 23, which is extremely powerful early-game
- Slowbro is much too slow to be very useful in-game as it will usually be moving last
In summation, Gyarados’ great stats and early-game availability make up for its shallow movepool. While Slowbro has good stats, movepool, and is available pretty early on, its terrible speed hinders its ability to easily and quickly take out opposing trainers in-game.