cvs -d:pserver:anon@sulfuric.graphics.cs.cmu.edu:/home/cvs_anon/cvsroot login

When prompted for a password, just hit <return>. Then,

  cvs -d:pserver:anon@sulfuric.graphics.cs.cmu.edu:/home/cvs_anon/cvsroot checkout reactor
  cd reactor
  cvs -d:pserver:anon@sulfuric.graphics.cs.cmu.edu:/home/cvs_anon/cvsroot checkout cl vl gcl

and follow the instructions in the README. 

This will give you the basic GCL libraries, as well as:
    slview
    slconv
    slrender
    imgview
    imgcomp
    anim

If you want a (possibly!) more stabl e version of the repository sources, add a -r STABLE to the checkout lines.