WordPress at a glance
function is not described

get_comment_guid() WP 2.5.0

Retrieve the feed GUID for the current comment.

No Hooks.

Return

false/String. false on failure or guid for comment on success.

Usage

get_comment_guid( $comment_id );
$comment_id(int/WP_Comment)
Optional comment object or id.
Default: global comment object

Code of get comment guid: wp-includes/feed.php VER 5.1.1

<?php
function get_comment_guid( $comment_id = null ) {
	$comment = get_comment( $comment_id );

	if ( ! is_object( $comment ) ) {
		return false;
	}

	return get_the_guid( $comment->comment_post_ID ) . '#comment-' . $comment->comment_ID;
}