Source

File: wp-includes/post-template.php

function get_the_guid( $post = 0 ) {
	$post = get_post( $post );

	$guid = isset( $post->guid ) ? $post->guid : '';
	$id   = isset( $post->ID ) ? $post->ID : 0;

	/**
	 * Filters the Global Unique Identifier (guid) of the post.
	 *
	 * @since 1.5.0
	 *
	 * @param string $guid Global Unique Identifier (guid) of the post.
	 * @param int    $id   The post ID.
	 */
	return apply_filters( 'get_the_guid', $guid, $id );
}