I have found that gDebugger 5.7 for Windows IS still available here:
http://files.gremedy.com/downloads/gDEBugger-5_7.msi
I modified this URL from the one found at the top of this download page:
view-source:http://www.gremedy.com/downloading.php?platform=windows32
It may be possible to access the same version for other platforms via the same trick.
An old license file is available here:
http://www.geeks3d.com/20101207/3d-programming-gdebugger-advanced-opengl-debugger-now-free/
But it expired on Jan 31st, 2011.
