Remember, however, that Regigigas has 100 base speed and 160 base attack (paired with great-albeit-low BP attacking coverage, Knock+Slam alone is pretty solid though) which alongside phenomenal bulk makes it another league above stuff like Snorlax (slow, exploitable physdef, 50 less attack) or Porygon-Z (slower and frailer, but stronger STAB).
This thing has incredible stats, I think it would at least see usage with an neutral ability like Clear Body. If the low BP moves are really a concern just think of it as base 110 attack with strong attacks lol
Kyurem B has incredible stats but it isn't broken... err, wasn't for a while anyway. Multiple Megas managed to be fair despite abilities that effectively boosted base stat total (Medicham, Mawile etc). There's also three of the four Tapus that in 7th gen remaining OU despite getting a massive 50% STAB bonus. Raw stats isn't always enough to break something.
Regigigas's strongest two STABs are Mega Kick and Crush Claw. Crush Claw is fucking bad lol and Mega Kick is grossly inaccurate leaving it with Body Slam. If we pretend Body Slam is a 100 base power STAB then Regigigas's most powerful attack is hitting only as hard as a 128 base power STAB EQ would. Strong by almost every metric but far from overpowered.
Yeah it gets elemental punches and Drain Punch but they're all 75 BP or less. There's also Knock Off which can be as weak as 65.
It does get EQ / Stone Edge / Heavy Slam so it's certainly capable of covering its bases but non-STABed attacks are significantly weaker. No doubt ability-less Gigas would be very, very strong and if Gamefreak has any sense of balance would ban it. But I think it could maybe be fair in OU.
252+ Atk Choice Band Regigigas Body Slam vs. 252 HP / 248+ Def Hippowdon: 171-202 (40.7 - 48%)
252+ Atk Choice Band Urshifu Wicked Blow vs. 252 HP / 248+ Def Hippowdon on a critical hit: 210-247 (50 - 58.8%)
252+ Atk Choice Band Rillaboom Wood Hammer vs. 252 HP / 248+ Def (??? type) Hippowdon in Grassy Terrain: 262-309 (62.3 - 73.5%)
252+ Atk Choice Band Rhyperior Earthquake vs. 252 HP / 248+ Def Hippowdon: 183-216 (43.5 - 51.4%)
252+ Atk Guts Obstagoon Facade (140 BP) vs. 252 HP / 248+ Def Hippowdon: 189-223 (45 - 53%)
252 Atk Choice Band Weavile Triple Axel (120 BP) vs. 252 HP / 248+ Def (??? type) Hippowdon: 180-216 (42.8 - 51.4%)
I guess my point is its damage output is high but far from incomparable to other things in OU. For example the standard "Rocky Helm phys def regenerator + switch in" standard used to deal with Urshifu does just fine at taking on Banded Gigas. Of course you could run Mega Kick which has the advantage of insane power output but runs the risk of missing and doesn't grant para hax. Also you can run Sub + Lefties or something over Band but then you're giving up a LOT of power (Fire Punch sometimes only 4HKOs Skarmory!). Imo it's a bulkier and slightly faster but much weaker Urshifu with worse typing.
But if it's of any concern Gamefreak could ban it from VGC and Smogon can sit on it for 8 months before finally banning it from OU. I don't have an issue with Gigas getting a better ability.