// Changes Gravity Forms Ajax Spinner (next, back, submit) to a transparent image
// this allows you to target the css and create a pure css spinner like the one used below in the style.css file of this gist.
add_filter( 'gform_ajax_spinner_url', 'spinner_url', 10, 2 );
function spinner_url( $image_src, $form ) {
    return  'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7'; // relative to you theme images folder

}