Thank you both!!
Jessica, when I invoked the guix shell as you showed, it worked in that "(use-modules (gi))" succeeded. I need to learn more about including packages with the guix shell command. It's odd that I can get the latest guile to run without specifying it for guix, but I must have the other packages for gi to work .. . .
David, I was able to install G-Golf via guix, and that also worked with the "use-modules" portion of your "Hello, World!" example.
HOWEVER (alas), both examples fail on the "use-typelibs" line, with:
Typelib file for namespace 'Gtk', version '4.0' not found
In trying to resolve that error myself, I found that the guile manual has no entry for "use-typelib" in the Procedures Index. :-/ I'll submit that bug in a new email, after confirming there's not already a thread.
But both of you have shown me that what I originally wanted to do is, in fact, doable. Thanks again!