christophercole: Oh my. I'm afraid none of you know how GOG, games, or OSes work.
Welcome. It'd be nice if you didn't start with an insult though.
This is not a GOG problem. It is NOT GOG that supports a particular version of a distro; it's the game devs. If they choose to support 20.04, then you'll start seeing 20.04 in the list of supported OSes.
GOG has chosen to support specific distros, and where they can choose which distros to support, they can also choose which versions of said distro to support. Just as they can drop support for Windows XP at will and fix games for Windows 10 at will. (The games, as provided by their developers, may very well work on other distros and the developer may indeed support other distros, but if you contact GOG about an issue on a distro they themselves do not support, then they will pretty much tell you to f*off and you're better off contacting the developer directly. I've been there. Hell, I've gotten that "we don't support your Fedora" treatment from GOG support when I pointed out a bug that was affecting users on Windows too.)
I don't know how exactly the responsibility between the developer and GOG is divided but unless you have insider knowledge (who are you again?), I wouldn't rule out the possibility that GOG helps (or at least has helped in the past) with the packaging and testing of Linux releases to ensure they run on the specific distros that GOG has chosen to support. Have you ever looked at the supporting scripts, directory hierarchy and arrangement of a GOG Linux release? They tend to be very similar, unlike (say) random humble bundle Linux games from random Linux developers of yore. It's almost as if GOG is supporting them..
GOG can work with Windows to make old games run on new versions of the OS because Microsoft itself supports backwards compatibility. Ubuntu much less so.
GOG could do the work independent of Ubuntu. Linux supports backwards compatibility, so you generally don't need the distro to provide much for you. It doesn't matter much who provides the required deps as long as someone does. I wouldn't be surprised if there's already some community-maintained repo of 32-bit libs. So where GOG now lists the libs you need to install from the official Ubuntu repos, one day they could list packages from a third party repo.
So, there's your answer. Newer native Linux games will have no trouble with 20.04 LTS. Older Linux titles on GOG will only be guaranteed to work on what they were designed to work on back when native Linux game development peaked circa 2015: 18.04 mostly.
Packages changing in incompatible ways has always been a concern on Linux, and it is not just a 32-bit multilib specific issue. You can take a native 64-bit game from today and try it in 10 years, chances are if it depends on more than libc and a small library or three, something will have changed and you need to go fishing for old solibs. That's why the best way to ship a game on Linux is to ship the libs with it. Until that becomes the norm, native Linux games will keep breaking over and over again and
someone has to keep maintaining these games that GOG sells.
I expect in the distant future Linux will continue to be supported by GOG in exactly the same way, [..] Or GOG could choose to repackage all Linux games as their Windows versions, emulated in little Proton bundles. Either way, GOG can keep Linux support in the long-term without much effort
Wait wait waaaait, didn't you just start the post off by insulting everyone here, then telling that no it's not a GOG problem and that GOG doesn't support particular versions of a distro? And now, in a total reversal, you're saying that GOG could repackage everything and maintain Linux support. For just about any version of just about any distro!
I agree! GOG could! If they so choose to. The choice whether to (or how much to) support Linux is very much a GOG problem. (And no, Flatpak and the alternatives you mentioned are not at all a requirement for this to be true. GOG could pick up any game they have today, bundle the missing libs, and ship an updated installer. If they so choose. Most of the cases are indeed that trivial to solve.)
Until that day comes, remember the biggest differences between Windows and Linux, respectively, are that the latter is free and only takes 10 minutes to install completely. So, install 18.04 on a spare comp or on a partition, and BOOM, you've got a perpetual GOG Linux gaming machine, just like that old SNES or Genesis you might have lying around the house.
Yeah, you go have fun running 2020 hardware on a 2010 kernel. You can backport drivers for all eternity.
Your complaints were founded on an unfortunate misunderstanding and indeed accidental ignorance of the current state of Linux gaming as well as the underlying tech that future-proofs all GOG games as well as what "support" means in GOG's storefront context.
It's pretty obvious that you haven't kept up with with what's going on at GOG regarding Linux support since they started it in 2014. The complaints that GOG has kept their Linux staff to a minimum and aren't giving it any thought are very real. The complaint that Linux releases go missing because GOG does not bother support Linux with their software is very real. New games releases where the first thing someone in the comments asks "where's the linux version?" are a weekly recurring thing. And it's been going on for a long time.
Let me say it again: Linux isn't a priority for GOG. At all. They either fired or pissed off one of their few key people and judging by the decaying state of things, they haven't hired much more talent to replace him. Do you think nobody's ever mentioned Flatpak to GOG? Do you think they've never heard of it? I know that they have heard of it, because we have talked about it here in the forums. Back when they still communicated with the community.
Nothing will change if there's no will on GOG's part. And if there is will, they can indeed choose to repackage and fix games for modern distros, with or without Flatpak.
Now should GOG go back and fix old games that weren't packaged well to begin with? To be honest, I don't think it's worth it, because these things are trivial to fix (by the user himself) which is why I told the OP to help themselves. Still, GOG has a ton of work to do if they want Linux users to take their store seriously. The work should probably start with Galaxy and all the missing Linux versions.