This depends on where you bought it. If on Steam, you just download it directly from Steam.

If on GOG, I do believe you will need Heroic launcher to actually be able to download the game to your Steam Deck.

Steam Deck using Linux puts it above anything else in the handheld category. It’s truly a device you yourself can do anything with, with no limitations or hard locks.

Not to mention the sw and hw support which both are excellent.

Realising that there is absolutely no way for me to, for instance, make a local backup of saves / settings, not even a way to access them on my Switch is a very sobering experience.