The SteamWorks SDK, an optional component if you want to use steam features like achievements, Steam friends in multiplayer, or DRM, is proprietary. But you can use Steamworks without Proton or Proton without Steamworks.
The rest of Proton is open source and is at its core a distribution of wine + dxvk.
You'll see DLLs with names like Windows DLLs, but they come from Wine, not Windows, and are reimplementations.
A lot of "works in proton but not wine" is to do with specific configuration of runtime dependencies or wine options. You can probably make all these things run in vanilla wine if you mess around with CLI opts and config files, but if you want the steam like "just works" experience you probably want a layer on top of wine. Lutris is a popular fully open tool for games that does this without needing Steam specifically.
So you mean that if I can extract the required fine tuning of wine/vkd3d from lutris I could run some games which would not run on a vanilla configured wine/vkd3k, just by the flick of some configuration I could run, cyberpunk 2077? apex legend? big games like that?
This is this not using the plain and simple C vkd3d (dx12) from wine but valve c++ dxvk. I still dunno why valve has not the decency to backport that into vkd3d, at least we would not need an absurdely and grotestely massive and complex c++ compiler, but the worst: it is using a ton of copies of windows DLLs from winetricks (I am better off installing windows).
I guess the win64 gog galaxy client does update the game to its latest patch.