Redcar: A Programmer’s Text Editor Written in Ruby
Redcar probably isn't going to be your next editor, at least not yet. It's still in its early stages with version 0.1 only being released a week ago and you have to get it running from source (no binary distributions yet). Despite all of this, it already boasts compatibly with Textmate highlighting, snippets, and quite a lot of Textmate bundle commands. It's GPL2 licensed, too. The code is available in a Github repository.
Despite the focus on GNOME, it should be possible to get Redcar running on both Windows and Mac OS X as well as Linux, but I've personally found the dependencies for Ruby-GNOME2 to be an absolute beast to install on OS X. Installation instructions are provided, however, for the brave.
Redcar is already a good looking app (on Linux, at least) and even though it's in its early days, it provides an interesting case study for if you want to build a Ruby GUI app based around Ruby-GNOME2.