override gravatar profile on comment

add_filter( 'pre_get_avatar', 'override_gravatar_comment_profile', 99, 3 );

function override_gravatar_comment_profile( $avatar, $id_or_email, $args ) {
    if ( is_object( $id_or_email ) && isset( $id_or_email->comment_ID ) && 55 === $args['size'] ) {
        return get_avatar( $id_or_email->user_id, $args['size'] );
    }
    return $avatar;
}