Filters the Gravity Forms spinner and replaces it with a base64-encoded blank image

gravity_forms_filter_spinner.php

function gf_spinner_replace( $image_src, $form ) {
	return  'data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7'; // relative to you theme images folder
}
add_filter( 'gform_ajax_spinner_url', 'gf_spinner_replace', 10, 2 );