I don't see why this isn't possible? Just link up Galaxy to GFN like it is with Steam & EGS.

Unless you go explicitly for a digital copy on any other storefront than GoG you'll end up with a GoG key anyway.
As far as I know the retail versions all come with one that can only be redeemed on GoG.

In regards to the GeForce streaming bollocks - it may be possible, but also may be in conflict with the DRM-free principle resp. the optional nature of the Galaxy client, in some way or another (authentification, Galaxy suddenly being mandatory for a feature other than multiplayer functionality, etc.).