GOSU is a great Ruby game framework.
This gem sets out to already provide basic objects and capabilities for use with gosu.
Take a look at
bundle exec ruby examples/tree_run/run.rb
bundle exec ruby examples/tank_vs_spaceship/run.rb
bundle exec ruby examples/cern_horror/run.rb
to get a feel what you can do.
(And then at the code for how easy it is ;) )