Google map comes partially, grey area comes instead of images from google server

This bug can occur if you are resizing the map's DIV. After resizing, try to call gmap.checkResize() function.

UPDATE 27/02/2020
There is no longer any need to trigger the resize event manually.

If you are using v3 try

google.maps.event.trigger(map, "resize");

Also take a look at here

Hi if you are using toggle in a div with a map container you call resizeMap in the function

associated with the trigger:
        $(".trigger").click(function(){
        $(".panel").toggle("fast");
        $(this).toggleClass("active");
         resizeMap();
        return false;

then resizeMap(); like this

function resizeMap()
{
google.maps.event.trigger(map,'resize');
map.setZoom( map.getZoom() );
}

don't forget to set map variable as global ;) cheers