[3/19/2009 Update (Aaron Lewis)]: This article is now obsolete and will be removed soon.  The GPHelper now retrieves all guild members from the roster, regardless of whether the person running the add-on has the mentioned option checked.

This happens if the person running the add-on does not have the option set in-game to display offline members.  While in game, open your guild window, then check the box to display offline members.  After you've done that, type /gph update in any chat window.  Exit the game, then upload the file as directed.