"The change to Keysyms seems to be a red herring; ..."

Fair enough. I didn't spend any time investigating that avenue, but was somewhat doubtful, hence: "... although, I don't know if that helps you directly ...".

From your follow-up post:

"Correction: Gtk3::Gdk::keyval_name does work."

Glad to hear it. I don't know if that's the best solution — as stated, "I'm not a user of Gtk3." — but at least it is a solution. Others may come up with something better.