I've seen that bug before, WU mistakes it for a unit software update, renames it accordingly as GUPDATE.GCD and places it in .System where it does nothing. Rename it to 006B090100.GCD and move it to the RemoteSW subfolder of .System then hook up your GTM60.

When you drag a software update file onto webupdater, it doesn't offer you that again because it's already loaded it to the device. It will offer you other updates of course after the specific file has been loaded.