Apparently I'm wrong about the gmap_markers.js location.  When using Drupal 5.xx, I kept gmap_markers.js in sites/all/libraries/gmap.  The behavior of the Gmap module appears to have changed with the upgrade to Drupal 6.xx and change from a private to public file system.  It seems I left a copy of gmap_markers.js in files/js and that was the copy being used, while I was assuming the module was automatically finding the copy in the third-party folder.  I'm still a tad nervous about leaving the file in files/js with read-write permissions so I'm changing it to read-only until I figure out the correct way to handle the situation.  I just have to remember to change the file permissions before regenerating the marker cache again.  Sigh!