If GOG Galaxy can handle alpha/betas by the time D:OS 2 is ready for that, then that will be supported, otherwise people can get Steam keys for the alpha/beta and OOG keys for the release.
With the D:OS Kickstarter the original plan was just to do alpha/beta keys on Steam, and then revoke those keys and give out Steam or GOG keys for the release. Since most people wanted Steam keys, though, just before release it was decided the beta keys would become the full game, and people who wanted GOG keys could revoke the Steam code and get a GOG key.