In Galaxy Client, under Konfiguration beta channel isn't selectable, so PreRelease seems not to be playable under GoG...

On KSP forums, they made it clear that any betas or pre-releases will be Steam only, because of how they can control stuff. They also promised to release it on GOG, so just wait for stable.

This I don't believe is widely available to devs yet anyway... only one game has used it as far as I know and that was the Witcher 3 which is owned by CD Projekt, same people who own GOG. It makes since they would be testing it.