gistfile1.php

<?php
/* 
 * Supported on GravityView 1.6
 *
 * Add this to your theme's functions.php file 
 * 
 * Replace the string 'MY NEW LABEL' for your own
 */


add_filter( 'gravityview_widget_search_filters', 'my_gv_change_search_filters', 10, 2 );
function my_gv_change_search_filters( $search_fields = array(), $instance ) {
	foreach( $search_fields as $k => $field ) {
		if( !empty( $field['key'] ) && 'search_all' === $field['key'] ) {
			$search_fields[ $k ]['label'] = 'MY NEW LABEL';
			break;
		}
	}

	return $search_fields;
}