It doesn't matter if it can gigantamax when it evolves, because it can't evolve at all. Gigantamax Meowth, Eevee, and Pikachu cannot evolve. In case you would like to have a persian anyways, you could breed it in the nursery, with the meowth holding an everstone so the egg won't hatch into a Galarian Meowth.
Source: Experience and Knowledge