Fixed the problem, it may caused by the javascript file. Here is what I have tried:

1. Open the file of "/sites/all/modules/gmap/gmap.module"
    

2. Find the function "_gmap_doheader()"
    

3. Find the line begin with "drupal_set_html_head('<script src..."
    

4. 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;
   

5. Upload the file to /sites/all/modules/gmap/

6. Flush all caches

I fixed the problem; it was caused by the JavaScript file. This is what I have tried:

1. Open the file of "/sites/all/modules/gmap/gmap.module"

2. Find the function "_gmap_doheader()"

3. Find the line begin with "drupal_set_html_head('<script src..."

4. 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;
      // …
    }
   

5. Upload the file to /sites/all/modules/gmap/

6. Flush all caches