Gtkradiant is a total mess, and the python install script from the SVN revision I tried some weeks ago was simply broken (it was absurdly assuming the OS was Windows and failed to install because radiant.exe wasn't there). I could compile it, though, but I ended up using an old working copy.
You may as well want to check zeroradiant, in the same site. Here it built and installed properly (I haven't configured it, so I don't know how well it plays).
Their mailing list:
http://zerowing.idsoftware.com/cgi-bin/mailman/listinfo/gtkradiantIRC channel (I think they're focused on zeroradiant now):
irc.telefragged.com #qeradiant
Zeroradiant site:
http://www.qeradiant.com/cgi-bin/trac.cgi/wiki/ZeroRadiantGood luck