By the way...
I have only tried out RubyGame a little bit. I checked out the rdocs you linked to.
Gosu only has a few basic classes, which provide a lot of utility in a fairly simple framework. This gives a lot of versatility allowing you to make your game features the way you want them.
There is also a gem which builds on top of Gosu, called Chingu, which has provides more built-in classes and methods, but has less overall versatility.
In case you are interested in taking a peek, Chingu's rdocs are located at:http://rdoc.info/github/ippa/chingu
Some people find Gosu easier to work with, and some people find Chingu easier to work with. I think they're both fantastic. Chingu has a few significant differences, but it is nice in that it has a lot of built-in extras. Like Gosu, it comes packaged with a lot of working examples, and can be installed with a simple "gem install chingu".
There are also several additional gems which beef up Gosu's repertoire, adding things like shaders and detailed pixel-level collision detection. I haven't tried them out too much yet. They are used in some of the more robust games on the Showcase forum.