function gforms_tag_with_name( $user_id, $feed, $entry, $password ) {

    $user = get_userdata( $user_id );

    $tag = $user->first_name . ' ' . $user->last_name;

    wp_fusion()->user->apply_tags( array( $tag ), $user_id );

}

add_action( 'gform_user_registered', 'gforms_tag_with_name', 10, 4 );