There are 2 ways of doing this:
Export to .md3 then import into GtkRadiant, this is what you would usually do for models. e.g. trolleys, statues etc
Export to .map then open in GtkRadaint, this would be suggested if you want to load a Blender map into GtkRadiant.
I don't have links to these scripts, but if you go here:
www.blenderartists.com, and do a search in the python scetion you should find plenty.