It has just been released on steam. Will there be some way to incorporate it to the GOG version?

im also hoping this mod/expansion comes to gog

Yeah we need it.

Agreed. Dear GOG team, could you please add the Downfall MOD/DLC?
Also, please update the game to the latest version.

The only updates of consequence between the GOG version and the latest one on Steam are some 32 bit compatibility and new language packs.

I just send an email to the devs to ask if they could take a look to see if something could be done to enable us to play Downfall.
If I have an answer, I'll let you know !

Edit : Devs answered me and unfortunately, they won't be able to bring it here, because moddersmade it available only on Steam workshop :/

well you can still probably make it work , I managed to grab some workshop mods and plug them on my GOG Slay the spire and the seems like the steam verison besides language packs and compatibility also just includes steam deck support which shouldn't impact many mods (or atleast the ones I use even though the mod manager warns about version mismatch.)