It's not that it's not for non-programmers; it just doesn't try to provide a fancy interface of buttons, check buttons, and what-not that would have to be custom-tailored for each file type... If you were to open a .utc file, you'd see wildly different information, but at its center, the file formats are the same.
The key for editing the file is the labels themselves. In the case of .gui files, you can look at the tag for an idea of what the control does.
Currently, this is how you edit .gui files.
However, here's my cheat sheet that I've worked up so far while working on a project for a while now:
GUI_Sub-sections.txt
As to your last comment, that was an ancient program that I don't believe was ever publicly-available; more of a debugging aid for the programmer of K-GFF.