Repeat Balls do not work on Gigantamax Pokémon. A Repeat Ball's effect is based on whether you have already caught the specific Pokémon species in your Pokédex. However, when a Pokémon Gigantamaxes during battle, it is still technically the same species—just in a temporary transformed state. The issue is that Gigantamax Pokémon are typically encountered in Dynamax Adventures or special raid dens, and the mechanics don't allow Repeat Balls to register an advantage in these scenarios.
The Repeat Ball works by checking your Pokédex to see if you've previously caught that species. While you may have caught the base form of the Pokémon before, the Gigantamax mechanic doesn't change how the catch mechanics operate—your Pokédex entry for the species remains the same whether you're encountering it in normal form or Gigantamax form.
In practice, trainers find that other Poké Balls are more effective for Gigantamax encounters. Timer Balls become more effective the longer the battle lasts, which is often useful in extended raid battles. Ultra Balls provide a consistent boost regardless of circumstances. Dusk Balls are highly effective in caves and dark environments where many raid dens are located.
If you're specifically hunting for Gigantamax Pokémon you've already caught before, you're better off relying on these alternative ball types rather than betting on the Repeat Ball's mechanics.