Dynamax and Gigantamax are essentially the same mechanic. If a pokemon doesn't know any attacks of its G-Max move's type, the difference is purely cosmetic. Banning Dynamax while permitting Gigantamax just sounds like a complex ban with extra steps.
Allowing Gigantamax means specs Toxtricity will be able to break choice lock at any time. We will have to deal with Hatterene setting up a Trick Room or Calm Mind and going wild with G-Max Smite, Max Mindstorm, and Max Flare. We will almost certainly have to ban Gengar to Ubers. Melmetal won't even get a chance at staying in OU when it's released. There will be more unnecessary bans once the next game drops and we get Gigantamax Garchomp or whatever. And let's be honest, half of the Gigantamax mons will just end up in UUBL anyway.
Some players might suggest tiering Gigantamax formes separately from base formes, similarly to how Megas were tiered. However, this would lead to double tiering. Take a future OU staple like Corviknight, for instance. In a Gigantamax-only OU, there is no downside to running Gigantamax Corviknight over the base forme, even if you're using Corviknight as a wall and Gigantamaxing your Hatterene in most battles. Regular Corviknight would see zero usage in OU and drop to UU, even if Corviknight were mostly being used for its base forme in OU. Double tiering couldn't happen with Megas because there was an actual downside to running the Mega (such as Garchomp losing its Rocky Helmet or Z-Crystal).
We also have to consider the impact on console battles. The distinction between Dynamax and Gigantamax is not entirely intuitive on console. Gigantamax mons are also difficult to obtain. I can imagine players finding it very unfair when they get swept by the rare Gigantamax Toxtricity that their opponent caught (or hacked), but when they try to Dynamax their own Toxtricity in response they're told that's not allowed.
Allowing Gigantamax means specs Toxtricity will be able to break choice lock at any time. We will have to deal with Hatterene setting up a Trick Room or Calm Mind and going wild with G-Max Smite, Max Mindstorm, and Max Flare. We will almost certainly have to ban Gengar to Ubers. Melmetal won't even get a chance at staying in OU when it's released. There will be more unnecessary bans once the next game drops and we get Gigantamax Garchomp or whatever. And let's be honest, half of the Gigantamax mons will just end up in UUBL anyway.
Some players might suggest tiering Gigantamax formes separately from base formes, similarly to how Megas were tiered. However, this would lead to double tiering. Take a future OU staple like Corviknight, for instance. In a Gigantamax-only OU, there is no downside to running Gigantamax Corviknight over the base forme, even if you're using Corviknight as a wall and Gigantamaxing your Hatterene in most battles. Regular Corviknight would see zero usage in OU and drop to UU, even if Corviknight were mostly being used for its base forme in OU. Double tiering couldn't happen with Megas because there was an actual downside to running the Mega (such as Garchomp losing its Rocky Helmet or Z-Crystal).
We also have to consider the impact on console battles. The distinction between Dynamax and Gigantamax is not entirely intuitive on console. Gigantamax mons are also difficult to obtain. I can imagine players finding it very unfair when they get swept by the rare Gigantamax Toxtricity that their opponent caught (or hacked), but when they try to Dynamax their own Toxtricity in response they're told that's not allowed.