I'm basically starting out with ruby from scratch and forgot how much I loved it, have a hate relationship with RM but that has it's own flaws and prefer just coding than using a stripped version of a language.

Now down to business, there's 2 libraries I'm looking at and that's Gosu(the 2D game framework for both C++ and Ruby), and SMFL which supports C++, Ruby, Python, and a few other languages.

At the moment I'm currently testing gosu as it looked good and had 3d acceleration and was already made for a game, smfl is also a great choice because it has everything you need and want out of the box and isn't limited to basic 3d as in PS1/early ps2 3D or HD 2D but it's capable of going as far as your computer can handle. And now for a 2D game I'm planning on working on well any of them would be fine.

This is my issue with Ruby. Its slow. every one knows that. I believe it has to do with the GC mechanism, but I am not sure. ask any ruby developer and they will tell you its slow. I would personally not use it to build a game from scratch, how ever I would use it to add scripted events, small events, and I would use C++ as the bottom back end language.