Power Whip has given Gyarados a lot of potential as it can blast past bulky Waters with ease now. With the right team support a DD Gyarados can be pretty scary. Pick the right targets to trade with and it opens up multiple ways for the other teammates to take the game. It does need a lot of support and this team is definitely build to facilitate Gyarados as well as possible. Wacan Berry is great to get a potential set up turn, or live a Rotom Wash Tbolt to KO back with Power Whip.
Arcanine fits perfectly for supporting Gyarados. Altho I do think it needs an aggressive approach. Howl or Will -o Wisp are imo just too slow for this team. Arcanine can blast Steel and Grass types that would give Gyarados trouble with FB, and you do need FB for plenty of power. WoW isn't missed as much with the double Intimidate core. Hidden Power Ice is teched in to punish potential Salamence and Hydreigon switches. Salamence in particular loves switching in on Gyara/Arca as it doesn't fear much from this duo save from a stray Ice Fang. With the speed EVs Arcanine can outspeed and OHKO Rotom Mow, while Gyara itself can threaten Wash for Arcanine with Power Whip.
Ferrothorn finishes the FWG core, giving great defensive synergy and bulk to the team. Ferro really improves the Rain match up and also gives hard TR teams a hard time with Curse. With a Curse up and Intimidate support, even Fighting types can have trouble breaking through Ferrothorn easily.
Garchomp adds a much needed Ground type, giving the team more key resistances to Electric and Rock Moves. Again combined with Gyarados this duo can beat a lot of match ups with their coverage and power alone. Yache Berry helps bait mons like Jolteon into HP Ice, or can give that but of longevity against things like P2 and Gastrodon. The attack EVs allow us to 2hko Metagross with EQ, with the rest dumped into SpA.
Tyranitar is here as another anti weather and TR mon. With Taunt it can break through many bulky teams, which this team could otherwise really struggle with. It also helps us keep P2 chipped and doing little damage, a very important job on this team. The Spdef EVs help prevent a Download boost and the speedcreep can allow us to Rock Slide flinch opposing Ttar in a pinch. Activating Sand Veil for Chomp is an added bonus.
Scarf Toge adds some much needed speed to the team and can deal with pesky Fighting types that can give Ttar and Ferro trouble. It also very much enjoys Electrics being taken out. Which Wacan Gyara and Yache Chomp help facilitate. Trick gives us amazing utility against TR or even TW teams.
Team Weaknesses: With no fighting type in the team, P2 can be a real menace. Tricking or Taunting it is really the best bet against it. Ferrothorn also does not care much at all about anything P2 can do. Ferro + Togekiss for example can cover each other well when P2 is on the field, as the opponent has no easy way of getting a fighting type in, and the most common fire type Chandelure gets clean 2HKO'd by Air Slash. Tyranitar prevents P2 from doing anything threatening with Taunt and can chip it down for Chomp or Gyara with Sand.
There is also no perfect way to deal with TW or Fake Outs. But the team carries a lot of Protect to try and play around these strategies, so there should always be options.
Other Ideas: While building I did notice that Gyarados pairs very well with an offensive Ice Type. In this team I couldn't give it a good fit. But I do think that Wfall + Power Whip Gyara partners extremely well with things like Mamoswine and Abomasnow.