Does it now support the "docklets" codes and companion app?
I think it has been posted here in the thread - or maybe it was another one. The only possibility to redeem a docket on Steam would be to link your Steam account to the devs website which is something most GOG users don#t want to do anyway since it is DRM. Part of the dockets however have been turned into DLCs here on GOG but not all of them. Don't know anything about any companion add so I cannot say anything about it.
It was me who first reported this. The "10 in 12" DLC's were all added, save the skins that were added through dockets.
The problem with the dockets with gog is that you can't link your gog account, so we don't get these skins, nor other promotions and stuff provided.
The companion app does not work (same reasons, IIRC) with the gog version at all.