Proton itself is open-source[1].
If someone wanted to package up standalone Proton binaries for a Linux distro, then I don't see any particular barriers that would prevent that.
On GoG's part, they do provide the ability to just download a game with a web browser (the old-fashioned, DRM-free way). From there, I can manage the games I that own in any way that I choose.
Thus, I'm simply not seeing a problem here that needs solved. I already have the freedom to do whatever I want.
Which part of this situation is broken, do you suppose, and why does GoG in particular need to fix it?
[0]: https://boilingsteam.com/valve-breaks-the-shackles-of-proton...
What does a person buy when they buy a game from GoG?
In my own experience with buying from GoG: What I buy is a copy of a Windows installer, and [often] a Steam code.
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If I then want to do something on my Linux box (or my Amiga 500) with that Windows installer, then: That's on me.
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I don't think that they owe me anything else here. YMMV.