Ok, let's blow away the 3.1.8 gem (because we had some problems with it) and reinstall it before we do anything in particlular.

gem uninstall gtk2
gem clean
cd /Users/me/.rbenv/versions/2.2.5/lib/ruby/gems/2.2.0/gems/
rm -R ./gtk2-3.1.8
cd /Users/me/.rbenv/versions/2.2.5/lib/ruby/gems/2.2.0/extensions/x86_64-darwin-15/2.2.0-static
rm -R ./gtk2-3.1.8
cd

If there are any errors up to this point, let me know. If not:

gem install gtk2

Then try the game.