Change the spinner image & show it above a semi-transparent layer

I don't know if this helps you...

You can put this in your style.css with the relavant path to your new spinner.gif

img.gform_ajax_spinner {
	position: fixed !important;
	z-index: 999999;
	left: 0;
	top: 0;
	right: 0;
	bottom: 0;
	display: block !important;
	overflow: hidden  !important;
	width: 100% !important;
	height: 100% !important;
	background-color: rgba(0, 0, 0); /* fall back */
	background-color: rgba(0, 0, 0, 0.7);
	background-image: url('images/preloading.gif');  /* path to your new spinner */
	background-repeat: no-repeat;
	background-size: 60px 60px;
	background-position: center center;
}

And then you have to replace the spinner.gif with a transparent 1px x 1px gif - but replace spinner.gif with the same file name.

This is located in your images folder in the gravity form plugins folder.

I have not tested it in all browsers and I'm not sure if it recommended to change the current spinner.gif incase it's used in the admin or else where. Plus if you do an update you will have to remember to replace it again with a transparent gif.

I'm sure someone could write some jQuery to edit the attribute of the spinner img to a blank.gif in your theme images folder.

OK, if you put this in your theme functions...

add_filter("gform_ajax_spinner_url", "spinner_url", 10, 2);
function spinner_url($image_src, $form){
    return  get_bloginfo('template_directory') . '/images/blank.gif' ; // relative to you theme images folder
}

or if it specifically for form ID 6, add the ID number after gform_ajax_spinner_url like this...

add_filter("gform_ajax_spinner_url_6", "spinner_url", 10, 2);
function spinner_url($image_src, $form){
    return  get_bloginfo('template_directory') . '/images/blank.gif' ;
}

And then if you use my original css in your style.css ( in my example below I have specified my class to form ID 6).

#gform_wrapper_6 img.gform_ajax_spinner {
    position: fixed !important;
    z-index: 999999;
    left: 0;
    top: 0;
    right: 0;
    bottom: 0;
    display: block !important;
    overflow: hidden  !important;
    width: 100% !important;
    height: 100% !important;
    background-color: rgba(0, 0, 0); /* fall back */
    background-color: rgba(0, 0, 0, 0.7);
    background-image: url('images/preloading.gif');  /* path to your new spinner */
    background-repeat: no-repeat;
    background-size: 60px 60px;
    background-position: center center;
}

Give it ago but otherwise see what the experts can suggest.

See documentation here.
http://www.gravityhelp.com/documentation/page/Gform_ajax_spinner_url