Taken from Gist here: https://gist.github.com/mannieschumpert/8334811

Filter the submit button in Gravity Forms to change the <input type=”submit> element to a <button> element.

There’s an example in the Gravity Forms documentation, but it lacks the proper code to show your custom button text, AND removes important attributes like the onclick that prevents multiple clicks. I was trying to solve that.

function gf_make_submit_input_into_a_button_element($button_input, $form) {

  //save attribute string to $button_match[1]
  preg_match("/<input([^\/>]*)(\s\/)*>/", $button_input, $button_match);

  //remove value attribute
  $button_atts = str_replace("value='".$form['button']['text']."' ", "", $button_match[1]);

  return '<button '.$button_atts.'>'.$form['button']['text'].'<i class="fa fa-refresh"></i></button>';
}
add_filter('gform_submit_button', 'gf_make_submit_input_into_a_button_element', 10, 2);