Some Gigantamax forms:
Torterra, Infernape, Empoleon, Spiritomb, Pachirisu (launch window distribution only), Lucario, Garchomp, Rotom, Unown

HMs are replaced with items that possessed by Rotom (like the Rotom Bike in Sword & Shield) and will be given to you by gym leaders. They are as followers:
-RotoSmash from Roark
-RotoCutter from Gardenia
-RotoCopter from Maylene
-RotoClear from Crasher Wake
-RotoBoat from Fantina
-RotoPush from Byron
-RotoClimb from Candice
-Volkner upgrades the Rotoboat to allow you to climb Waterfalls

Sinnohan Forms
-Toxel (given to you in a mystery egg from the day-care)
-Roggenrola line (Ice type)
-Lycanroc (evolve Rockruff at Mt Coronet)
-Silicobra/Sandaconda (Fire type)
-Sizzlipede/Centiskorch (Ice/Bug type)

Other details:
-Pokémon following you restricted to amity square and the Battle Zone
-Battle Zone is now a wild area
-Battle Frontier unlocked after the Giratina post-game story
-Underground now online too, with super secret bases like ORAS
-Spiritomb is in the postgame now due to Dynamax and Gigantamax being postgame only
-The Valley Windworks Drifloon that appears every Friday now has 3 guaranteed perfect IVs and its hidden ability
-Same for the Old Chateau's Rotom (Sidenote: Rotom's Gigantamax form will become a meme)
-Team Galactic have been slightly redesigned, but not much has been done with them
-The only way to get Gigantamax Lucario is from the gift Riolu
-Oak's Letter, Member's Card and Azure Flute are obtainable in the postgame
-Gigantamax Unown in a postgame event after you catch all 28 currently existing forms at the Solaceon Ruins
-SinJoh Ruins are not forgotten...