You can add non-steam games to Steam.

did not know, i dont use steam
lucky you tho good luck

So my DDE just showed up in GOG, both in Galaxy and on the BG3 game page in my library, but I also don't show as having purchased the DDE upgrade on GOG. This kinda makes sense, since I didn't purchase the upgrade. It was a bit confusing, though.

For those like me who are wondering WTF: odds are you have the upgrade at this point, but you may need to exit and restart Galaxy for the DLC to show up. I guess it's not great at refreshing data or something.