I know a few of tools of this sort, but they're all LJ focused and I'm not sure if they'll handle the GJ system even though the source structure is the same.

Anyway, the thread unfolder is a Greasemonkey script, and the backup tool is called "LJ-Archive," I believe.

This very long and comprehensive post by elke_tanzer might have more information (it dates from Strikethrough 2007), and if not ... she's crossposting and migrating and has always been pretty incredible as a resource, so there might be something elsewhere in her files.