This one time, I was playing National Dex AG with my Zygarde. It used Dragon Tail as the enemy dynamaxed, the foe took damage but didn't get dragged out. Unfortunately that battle is lost in history. However, this is what Bulbapedia says
When used on a Dynamax or Gigantamax Pokémon, Circle Throw will deal damage but not force the target to switch out.
So, Circle throw does fail against a Dynamaxed/Gigantamax Pokemon
Hope this helps!