First: Follow this tutorial:
https://groups.google.com/d/msg/rubyinstaller/vMR5mQVA_88/aZqLyjyQiSYJ (by Tomasz Błajek).
You will get an error with last step (building the gem). At this point you should have this directory tree:
(folder) doc
(folder) examples
(folder) ext
(folder) lib
(folder) test
(file) metadata
(file) Rakefile
Open "metadata" and add after line 13:
licenses: []
Then run:
gem build metadata
After you will have to download and install Glut to the devkit. Download it here:
http://user.xmission.com/~nate/glut.html (the bin option).
Unpack and move 'glut.h' to "<devkit path>\mingw\include\GL", 'glut32.lib' to "<devkit path>\mingw\lib" and 'glut32.dll' to "C:\Windows\System32".
Now run:
gem install ruby-opengl-0.60.1.gem
You shouldn't get any errors.
(I'm now testing it with rbSFML.)