(from the FAQ page for the new policy, https://support.gog.com/hc/en-us/articles/360006129837?produ... )
I'm intrigued by the internals here. GOG does indeed sell some things both bundled and individually, like the "Hollow Knight + Soundtracks" bundle compared to "Hollow Knight (no soundtracks)" / "Hollow Knight Official Soundtrack" / "Hollow Knight Gods & Nightmares DLC Soundtrack".
It is not obvious to me why an account that purchased the bundle should be treated differently from an account that purchased the three individual offerings. (Well, in this case, the bundle is cheaper, but that's not always true.)
Amazon has this problem too. If you buy an mp3 from them, they'll discount the cost of the digital album that mp3 was technically offered from. If that album exists as multiple SKUs, only one SKU will see the discount. Be careful! And Kindle MatchBook will likewise only match if the editions of the ebook currently offered and the physical book you purchased coincide. Was a new edition released with a new foreword? You need to buy another paper copy if you want to be re-eligible for MatchBook. Even if there's only one ebook offering, for the current edition -- even if the old version of the ebook is automatically updated to the new version -- your old purchase stops qualifying you for MatchBook when the new edition comes out.