If you haven't even found Duraludon yet, check the Polar Biome in the academy terrarium. They're fairly rare Pokemon, so you may need to leave and come back to reset the wild 'mon roaming around.

All four Gigantamax Pokemon come with their own exclusive G-Max moves, which deal tons of damage and have other effects. Duraludon’s G-Max Depletion reduces the PP of the last move its target ...

Pokemon Scarlet and Violet's DLC, The Indigo Disk, adds Duraludon and gives them a new evolution, Archaludon. Duraludon is a steel/dragon type, which makes it resistant to dragon-type attacks ...