Heroic has some issues (i.e. Stellaris) that require some sort of annoying work around that doesn't really work sometimes. GOG could not have those issues.

Heroic is limited to just GOG and Epic, while GOG Galaxy on Windows is already trying to be a sort of everything launcher. This means GOG Galaxy on Linux would be more like a replacement of Lutris (which is no longer being developed) than Heroic.