Change a Gravity Forms submit input to a button.

wp_gravity_forms_submit_button.php

<?php
/**
 *  Change a Gravity Forms submit input to a button.
 *
 *  @param   string  $button  The string of the submit input html
 *  @param   object  $form    A Gravity Form form object
 *  @return  string           Our modified submit button string
 */
function wp_gravity_forms_submit_button( $button, $form ) {
    return "<button class='btn' id='gform_submit_button_{$form["id"]}'>{$form['button']['text']}</button>";
}
add_filter( 'gform_submit_button', 'wp_gravity_forms_submit_button', 10, 2 );
// add_filter( 'gform_submit_button_1', 'wp_gravity_forms_submit_button', 10, 2 );    // Specify a specific form ID