Talking about flatpak - I, personally, agreed, that something like this should replace current way, GoG distribute linux games (2 reasons - security (flatpak designed to be sort of sandbox either) and smarter support (Im talking about situations, when games with older libs packed, doesnt work on distros newer than ubuntu, and, coz they arent officially supported by GoG, you will need to spend dozens of hours in order to fix it. Using flatpak/snap/etc, will handle 99% of these situations - your games will work on any linux distro, which has flatpak/snap support). On other side - I understand GoG's position either - right now, nobody knows, what will survive this game: flatpak, snap or old but gold appimage. So - they just wait, coz its business and repacking packages costs time (and so - money to pay people, who doing so and testing it later).
Iirc, nothing stops you from packing GoG's blobs into flatpak by uself, if you wish it RIGHT HERE, RIGHT NOW