alt Hacker NewsGPTK is mostly a bunch of developer tools for converting to Metal, and the closest it gets to anything like Proton is an "evaluation environment" that is nothing close to Proton's performance. Proton is mostly Wine, and Wine on macOS uses MoltenVK, so it's probably easier to just port Proton.
Direct3D -> Vulkan -> Metal is quite the translation layer sandwich, I wonder if that would have a meaningful impact on performance