Time after time I'm facing this issue with gmap markers. As mostly this happens when I'm doing something on the server side with file permissions, I decided to keep the notes

here is the problem

GMap is unable to save the marker bundle, so the markers will not work. Please check file system permissions of the public://js/gmap_markers.js!

Here is the solution

sudo chmod 755 /sites/all/files/js/gmap_markers.js