I've decided to give the GTK3 library a try for writing small GUI applications in Ruby on the Raspberry Pi. My two main reasons are that the Ruby GTK3 Gem was easy to install and, I believe, that GTK3 will benefit from the progressive move to Wayland in Raspian.
Installing the Ruby GTK3 Gem was simply a matter of a single command and a little patience. It took some time for all the native extensions to be built.
Once the gem was installed, the Ruby Gnome 2 Hello World worked with the minor change of requiring gtk3 instead of gtk2.
If I have the time, I'm going to see if I can come up with a simple Domain Specific Language that would make it easier for children to write GUI programs in Ruby.