/**
 * Filters the next, previous and submit buttons.
 * Replaces the forms <input> buttons with <button> while maintaining attributes from original <input>.
 * @param string $button Contains the <input> tag to be filtered.
 * @param object $form Contains all the properties of the current form.
 * @return string The filtered submit button.
 */
add_filter( 'gform_next_button', 'input_to_button', 10, 2 );
add_filter( 'gform_previous_button', 'input_to_button', 10, 2 );
add_filter( 'gform_submit_button', 'input_to_button', 10, 2 );
function input_to_button( $button, $form ) {
    $dom = new DOMDocument();
    $dom->loadHTML( $button );
    $input = $dom->getElementsByTagName( 'input' )->item(0);
    $new_button = $dom->createElement( 'button' );
    $button_span = $dom->createElement( 'span', $input->getAttribute( 'value' ) );
    $new_button->appendChild( $button_span );
    $input->removeAttribute( 'value' );
    foreach( $input->attributes as $attribute ) {
        $new_button->setAttribute( $attribute->name, $attribute->value );
    }
    $input->parentNode->replaceChild( $new_button, $input );
    return $dom->saveHtml($new_button);
}