foad01: There could be something which happened in the background. Maybe the studio itself doesn't exist anymore but there is still one person or a group who has the copyrights of the games.
MarkoH01: It does not work this way. GOG has a contract with a certain publisher and as long as the contract exists the publisher can allow GOG to put certain games on sale or give them away for free. IF the publisher is changing new contraccts need to be signed as well. In that case however the publisher on the gamecard would change as well - which did not happen. Without a new contract with the new publisher it is impossible for GOG to give away a certain game without getting in legal trouble.
I'm pretty sure foad's idea has merit. Remember when the Descent games were pulled from GOG because the publisher hadn't paid royalties (or whatever) to the developer? The developer hadn't existed for years at that point, except on paper, as a means for the owners of the games' code and assets to receive payments per their contract with the publisher.
Also, just because a game's developer and publisher fields say certain things, it doesn't mean that those things are necessarily 100% true. If I recall correctly,
Arx Fatalis used to be listed as being self-published on GOG, but it was apparently sort of "shadow-published" by Bethesda (which, by now, has been actually listed as publisher for a long time).
EDIT: Fixed an embarrassing misspelling.