Rob,
You can't currently configure the sort order ( ascending vs descending ) but to 'hard-code' the change, you can simply modify the grmGetRepositoryObjects stored procedure. Look for the following...

ORDER BY
  CASE @sSort
    WHEN 'Name' THEN a.Name
    WHEN 'Author' THEN a.Author
  END ASC,

Change it to this...

ORDER BY
  CASE @sSort
    WHEN 'Name' THEN a.Name
  END DESC,
  CASE @sSort
    WHEN 'Author' THEN a.Author
  END ASC,

If this is the only repository on your site and you don't mind that when the items are sorted by Name ( Title ) that they will always be sorted in reverse order, then this should do it.