has anyone ported this gui system to normal C?
I wasn't so much referring to the TempleOS GUI system there as the embedding, and first class support for bitmaps, and polygonal 3d graphics. It's not a complete solution either, these embedded items are still based on lines with regards to printing for example. Further why stop at bitmaps and polygonal 3d graphics? Literate programming is just the embedding of program source in a text document with ideally embedded typesetting. A slip-box (zettlekasten) is just arbitrary links between text documents within the documents. Why not have links to arbitrary items at arbitrary positions, notes linking directly to the podcast, blog, or book that they came from? Why not have links updated when a resource is updated? Why are there still second class objects?
Also if you actually look at the interfaces of TempleOS some I think are rather restrictive. If you look at the bitmap and 3d polygon construction software to me they are excessively rigid and ungraphical. If you're going to be using your mouse anyway, or even a drawing tablet, an interface based on the keyboard is not necessarily the best. I think something closer to Krita's interface is a better example of what a paint application should be. Although the GUI should still be dynamically and graphically configurable by the user at run-time in a manner universal on the system like Morphic on SmallTalk.
I don't actually know how GUI programming works in TempleOS, I get the impression that it wouldn't be graphical, and I rather think that should be an option. I do know that it provides an infintly less baroque graphical library in contrast to something like OpenGL, but I have done fairly little graphical programming, basically just blitting raster onto the screen, setting up an OpenGL context etc. I don't really know where we should go, but it does seem like it shouldn't be OpenGL or similar. Judging by the rest of the OS I wouldn't be surprised if Terry had some decent ideas we can learn from, along with many ideas we could do without.