Fast, hits hard, and the obligatory shiny, Gyarados is a good Pokemon in the tier. It's got some moveset issues (especially on the physical side), but play to its strengths and it will be the final nail in the coffin for the other guy's team.
Set 1: Mixed Cleaner
-Hidden Power Ground
Double Edge is a nice standard attack, getting nice neutral damage on most things. Hydro Pump is the obligatory water STAB, 2HKOing Nidoqueen and allows it to 1v1 Sandslash. Thunder is your electric move, giving the tier's waters a nice "surprise" and is its best option against Slowbro. Hidden Power Ground is for things that don't like ground moves, Electabuzz in particular. The aim of this set is to just try and kill everything it can. Its move coverage hits everything the tier for at the very least neutral damage.
A RestTalk set with Hydro Pump and Hidden Power Flying can always be a good option for a bulky attacker, although it is walled by a fair bit in the tier. Zap Cannon can be used in place of Thunder, although its low accuracy makes it a shaky trade. Hidden Power Flying can be used on the standard set instead of Hidden Power Ground to gain some more advantageous matchups against bug and grass types, although it does lose super-effective coverage on electric types. Substitute can be used in place of HP Ground to improve its matchup against electrics, as well as block paralysis.
[Checks and Counters]
Paralysis is a huge detriment to Gyarados, leaving it helpless against most anything in the tier, since its speed is the main driving point. Electabuzz in particular is a decent counter, as it can switch into HP Ground (emphasis on can, it's about a 75% chance to 2HKO) and retaliate with Thunderbolt.