Just a quick run down, not too in-depth...
Chansey: I think Fake Out and Skill Swap are usually goto picks on Imposter sets, the former for breaking Illusion (assuming no mega/primal evolve) and the latter to shut down an opponent then and there if you can't Imposter.
Rayquaza: Blue Flare over Fire Blast, it's a straight upgrade. Light of Ruin does not trigger Sheer Force, but it will trigger a hell of a lot of recoil.
Zygarde: Should be really running minimum speed for a pivot/Core Enforcer user.
Gigas: No STAB??? Any PH Gigas worth their salt needs Facade. Without it, you might as well be running something else. Your coverage has no offensive presence, especially since Giratina and Gengar, among others, resist it. Also, use Spiky Shield or Baneful Bunker over Protect. Or perhaps King's Shield. Also, Shift Gear over Dragon Dance.
Gyarados: Assault Vest Unaware is.... kinda useless as it'll get worn down and be unable to do its job to stop set-up users, especially multiple.. Gyarados offers almost no threat to defensive Pokemon without some sort of boost as well, so running four attacks + AV isn't particularly great.
Arceus: Still a normal type, so no STAB on Moonblast. Perish Song usually needs to be on a trapper or Unaware.
Overall: The team looks like its just random sets in a vacuum thrown together, as the defensive synergy seems kind of... eh. For example, you have a serious Fairy weakness and nothing to cover it. Offensive synergy isn't great either, as your best bet, Rayquaza, gets checked by Giratina if it has Core Enforcer, since Life Orb Light of Ruin recoil alone 2HKOs you and brings you well in range of a Core Enforcer KO while Gyarados only threatens it once with Knock Off. More importantly, the team seems to have no strategy or goal. I can't tell what it's trying to accomplish other than "these sets seem good, let's go."
I would recommend looking at some sample teams in the Central Resources thread here:
https://www.smogon.com/forums/threads/balanced-hackmons.3587475/#post-7094085
You can also find a "Setpedia" to get an idea on where to start with making sets here:
https://docs.google.com/document/d/...KkPWb51_ahdapkkdvPBdcNihXm5QTgRP8GXLaPTXM/pub
I'd recommend making a new team after that since anything competitive is going to be a complete overhaul of your current one at best.