According to protondb.com BG3 should run okay-ish on the Steam Deck, but with some tweaks. Just from a glance it looks like people skip the launcher after initial setup and lock the fps to 30-40ish. You can find more details here:

protondb

My man. Post the name instead of some random acronym that could be applied to about a million games. No one actually knows what you're talking about.

The early access is completely playable in Linux so it will work on the Steam Deck. Whether you'd want to play it on the Steam Deck due to its lack of native gamepad controls is another issue.

Previous Larion Studios games work on Linux, so even if it's flaky on launch, I suspect it won't be long before it starts working well

https://www.protondb.com/app/1086940 Always suggest checking protondb. But looks like the vast majority of people can play it in desktop Linux and SteamDeck with slight tweaking. I've been waiting for the full release before playing

There was a video when the Steam Deck was announced with the Larian CEO playing on a Steam Deck and being blown away by it, he mentioned that they would make sure it rund via proton on the deck.

Also BG3 was one of the games that was used in the video marketing and screen shots for the Steam Deck so I'm confident Valve will be putting their best efforts to work with Larian on this

it would probably help if you posted a full name of the game you're talking about, instead of assuming that everyone knows what you're referring to.