If you don't have the DLC, then yes, you must find a Gigantamax Pokemon in a den. They're quite rare outside of events, so good luck. If you have the DLC, you can apply Max Soup to your Corviknight to give it the Gigantamax factor. Alternatively, you can trade with someone and ask them to feed your Corviknight Max Soup, if you're not willing/can't get the DLC. Gigantamax cannot be bred or transferred from one Pokemon to another, so don't try.
Hope I helped!