Add class(es) to Gravity Forms buttons

gf_button_addclass.php

add_filter("gform_submit_button", "alter_form_submit_button", 10, 2);
function alter_form_submit_button($button, $form){
  if ($form["button"]["type"] == "text") {
    // (class=) followed by (single or double quote) followed by (anything that is not a single or double quote)
    $pattern = '/(class=)(\'|")([^\'"]+)/';
    $replacement = '${1}${2}${3} btn';
    $newbutton = preg_replace($pattern, $replacement, $button);
    if ( !is_null($newbutton) ) {
      return $newbutton;
    }
  }
  return $button;
}