It's safe to delete the .gz file.
Currently both the archive/unarchive command have difficulties when importing a course that was created with a different version of WeBWorK -- specifically when the database schema changes. (One could fix this by hand working directly with mySQL by adding a math112-fall-2007_set_locations table but it is a bit delicate.)
It's high on my list of things to rectify in the next release of WeBWorK.
Finding the time is the hard part.