AmethystViper: Bloodstained: Ritual of the Night - Gets updates much slower than Steam does (e.g. GOG players waited for months for their v1.17 update to be released, only to not fix one of the
major problems that breaks 100% completion), and the Soundtrack DLC is lower quality than the Steam version (
link). Although not directly related to Bloostained: Ritual of the Night, but thanks to Inti Creates and their adamant stance on anti-piracy, they screwed over the game's backers from ever getting a GOG/DRM-free version of Curse of the Moon.
Rune Classic - This game never got the v1.11 update that Steam did, which fixed some issues with users with DirectX 11-capable systems and Windows 10 systems (
NOTE: the update does NOT add DirectX 11 rendering to the game at all, it still uses Direct3D 7 like in the original releases, the update only fixed things like resolutions not appearing in the game's video settings).
Darksiders II: Deathinitive Edition - Did not get the beta updates that supposedly fixes crashes involving one of the game's main quests.
i think bloodstained is currently up to date tho, is it not? the 1.18 bloodless update came out on the 10th on both steam and gog i think.
but yeah, rune and darksiders both need their respective updates.