I feel like this and DRM are the two things people point out as GoG's benefits, but first of all GoG also sells you a license, so it's EXACTLY the same as Steam. The "difference" is in the other point, because without DRM games bought on GoG can be backed up and installed again without needing GoGs approval, but this ignores two HUGE caveats:
- GoG also allows DRM on their store, with some games having DRM for DLCs or extra content.
- Steam doesn't require DRM, so all games that are DRM free on GoG are most likely also DRM free on Steam.
All in all this means that in regards to ownership morally GoG is essentially the same as Valve, they just have better PR about it.
PS: To give credit where it's due, GoG is a lot more restrictive with what DRM it allows, and you can expect most of the game you bought to be playable without DRM there. However I feel that allowing me to play on my libre OS is a lot more morally significant than the difference between those two DRM stances.