I fixed the problem; it was caused by the JavaScript file. This is what I have tried:
Open the file of "/sites/all/modules/gmap/gmap.module"
Find the function "_gmap_doheader()"
Find the line begin with "drupal_set_html_head('<script src..."
Move it to the second line of _gmap_doheader(), so it may look like:
function _gmap_doheader() { drupal_set_html_head('<script src=xxx static $gmap_initialized = FALSE; // … }
Upload the file to /sites/all/modules/gmap/
Flush all caches