dev-ruby/ruby-opengl-0.32d has DEPEND="media-libs/glut" (among other things). However, there are two glut packages --- media-libs/glut, media-libs/freeglut --- both of which PROVIDE="virtual/glut". Unless there are compelling reasons to the contrary, ruby-opengl should DEPEND="virtual/glut". (I can test and make the change, if you like.)
If you want to test and make the changes, I'd be very happy :)
Done. Closing, almost before it was opened. :)