Fricking sweet!
Only one omission I've noticed from the Rdoc display: you can't navigate to files, in particular you can't navigate back to the main index (aka Readme) page, and although the RDoc page for Copyright exists at
http://www.libgosu.org/rdoc/files/COPYING_txt.html , it is (as far as I can tell) impossible to navigate there.
But those are minor quibbles. Overall, three cheers!