Panda Canvas is an educational 2D drawing canvas on top of Gosu and TexPlay.
It is created as a teaching tool for the Computer Club n.a. 8-bit Panda.
Panda Canvas is available as a gem:
$ gem install panda_canvas
Running an endless draw block is not possible on Windows. In a unix environment Panda Canvas uses fibers to pause the draw block in the Gosu game loop on flush. With tested Gosu and Ruby versions on Windows this results in a segfault. So, instead of fibers, the block must be run once and complete to capture all of the canvas calls before drawing on screen.
Take a look at the wiki.