http://www.mesa3d.org/faq.html#part4 says:

  3.4 Why does glGetString() always return NULL?

  Be sure you have an active/current OpenGL rendering context before
  calling glGetString.

So this should work:

  require "opengl"
  require "glut"

  GLUT.Init
  GLUT.InitDisplayMode(GLUT::DOUBLE | GLUT::RGB | GLUT::DEPTH);
  GLUT.CreateWindow("glGetString test");
  p GL.GetString(GL::VERSION)

And this seems to work, too, but I don't know if gluGetString has the
same limitation has glGetString:

  require "opengl"
  p GLU.GetString(GLU::VERSION)

Paul