Gastro Acid cancels the target's in-battle Ability. Therefore, the target's Ability will be ignored until it is switched out, even if it Transforms. It does not affect Abilities that only have an effect outside of battle, such as Illuminate. Furthermore, Gastro Acid does not negate Abilities that have an immediate effect at the start of the battle, such as Drizzle, because the Ability will activate before Gastro Acid can even be used. Gastro Acid cannot be used to deactivate the Ability of a Plate-holding Arceus. Steel-type Pokémon are affected by Gastro Acid.