However, I don't see good reasons to write yet another GUI
and widget toolkit entirely in Smalltalk.
Yes, quite so. It was instead low level X things that had prompted
me. Various window managers are a bit dodgy in various areas, like
event handling during drags, and I got the urge to try something
minimal myself. C is a bit too much like hard work for such things,
if any sort of configurability is going to exist, hence nosing around
gst (and guile-xlib and elk), despite not really being fluent in
smalltalk (or scheme).