gf_submit_button.php

<?php

/**
 * GravityForms Change Submit Input to Button
 *
 * @param array $button
 * @param array $form
 * @return string
 */
add_filter( 'gform_submit_button', 'gform_submit_button', 10, 2 );
function gform_submit_button( $button, $form ) {
  return "<button class=\"button gform_button\" id=\"gform_submit_button_{$form['id']}\" type=\"submit\"><span>{$form['button']['text']}</span></button>";
}