I'd like the point out that "Gigas requiring zero team support" is quite wrong. Which you already proved wrong yourself by mentioning the Core Enforcer switch-in. But, Gigas has very significant weaknesses that do require addressing in your team. Now, while Gigas has a number of possible sets, I'm gonna focus on its two that function reliably and consistently: PH Sleep Set-up and PH Sleep Shuffle.
The first issue is the Toxic Orb itself. If Gigas can't activate it, it sucks. Therefore, another Pokemon on the team must act as a lead since Gigas can't do so safely without running Spiky Shield, which the set-up set can't afford and the shuffle set doesn't want to do, as losing the Orb turn 1 to Trick or Knock Off is crippling. This also requires safe switch-in opportunities, usually via pivot. And finally on this point, Misty Surge gives Gigas a rough time, so it really appreciates a teammate who can remove it since it can't activate its Orb in Misty Terrain.
Second is Gigas is starved for moveslots. Every set must have Facade. If you're not running Facade, then you need to not be running the set on Gigas. Second, Gigas almost always has Spore to Imposter-proof itself and to give it the set-up opportunities it needs. Third, set-up sets run Shift Gear (occasionally Shell Smash, but its riskier and less consistent), which is slow, and shuffle sets choose between D.Tail, Circle Throw, or Whirlwind. This leaves it one moves, ONE move, to play with, that's usually taken by Knock Off. As such, it's always going to be walled by something. PH/Magic Bounce Giratina, Zygarde, Registeel, Solgaleo, and Celesteela are all common walls that Registeel cannot usually beat and struggles to beat with other abilities like Fur Coat, Unaware, Prankster, etc. It also has less common counters in Pokemon like Regirock. And especially Ferrothorn, who its usual sets flat out can't touch. It can't run Fighting coverage for the Steels because Imposter will have a 50% chance to OHKO if it sets up enough. It can't run Sacred Fire because burning opposing Imposter is a death sentence. V-Create cuts the bulk and speed Gigas desparately needs to function. And all of those options leave it walled by Giratina anyway, who it really has nothing good except Power Trip... which pretty much requires Shell Smash to use reliable and Imposter says SPEED TIE TIME! again. Gigas needs A LOT of boosts to break most/all of these walls with its common moves as all of them can switch in with impunity and force Gigas out.
Second and a half, Shedinja. Endeavor + Extreme Speed KOs through PH and most Gigas sets can't touch it.
Third, Gigas provides anti-team support by limiting what status moves its teammates can use and, if they run them anyway, how they can use them. Toxic Spikes are flat out a no on any team using Gigas and Baneful Bunker and Toxic are questionable decisions too. Gengar can't run Poison coverage safely, especially if the Imposter is copying someone else. Moves like Scald, Will-O-Wisp, and Sacred Fire become dangerous to use because a burned Imposter will Facade Gigas' face in short order. Paralysis through Nuzzle and Glare are the safest non-Sleep status to use since the Imposter is a little less likely to bash Gigas' face in, but its still dangerous because boosting Facade and removing sleep vulnerability is what makes Gigas cry when facing down an Imposter.
Fourth and finally, Gigas' set-up is slow. Once its going, it can be dangerous if the appropriate checks/counters are out of the way. But, it requires two turns: sleep opponent and then set-up. As this is super predictable, a competent opponent will respond right away and have their check/counter in the field in no time. Gigas also simply cannot set-up in front of a number of attackers. Specs -ate Boomburst Ray and Diancie outspeed and OHKO. Xtwo and Blazekin OKHOes with its STAB on most sets. Ytwo OHKOs with a number of its sets. And that's just off the top of my head. There's also a lot of others who can chunk Gigas hard and then swap to something not-slept and immune to sleep to finish it off. And, with Gigas' only recovery being PH, its quite vulnerable when its HP is low unless the controlling player has full control of the game's momentum.
Also, niche/rare checks in Knock Off + Purify, Trick Lum, Trick Black Sludge, Infestation/Magma storm + Leech Seed, etc. Most of those aren't relevant on ladder, but worth noting if someone counter-teams in a tournament.
Now, yeah, Gigas is really strong currently, but it DEFINITELY needs some team support, especially since it has a number of counters that may very well start cropping up since teams don't need to spam Fur Coat so hard anymore.