MySQL n'a qu'à bien se tenir ... :

SELECT 
	P.post_id, P.blog_id, P.user_id, P.cat_id, P.post_dt, P.post_tz, 
	P.post_creadt, P.post_upddt, P.post_format, P.post_password, 
	P.post_url, P.post_lang, P.post_title, P.post_excerpt, 
	P.post_excerpt_xhtml, P.post_content, P.post_content_xhtml, 
	P.post_notes, P.post_meta, P.post_status, P.post_selected, 
	P.post_open_comment, P.post_open_tb, P.nb_comment, 
	P.nb_trackback, 
	U.user_name, U.user_firstname, U.user_displayname, U.user_email, 
	U.user_url, 
	C.cat_title, C.cat_url , 
	M.media_file, M.media_id, M.media_path, M.media_title, 
	M.media_meta, M.media_dt, M.media_creadt, 
	M.media_upddt, M.media_private 
FROM dc_post G 
	INNER JOIN dc_meta GM on GM.post_id = G.post_id 
		and GM.meta_type="galitem" 
	INNER JOIN dc_post P on GM.meta_id = P.post_id 
	INNER JOIN dc_user U ON U.user_id = P.user_id 
	LEFT JOIN dc_category C ON P.cat_id = C.cat_id 
	INNER JOIN dc_post_media PM ON P.post_id = PM.post_id 
	INNER JOIN dc_media M on M.media_id = PM.media_id 
WHERE 
	P.blog_id = 'default' AND P.post_type = 'galitem'
	AND G.post_type="gal" AND G.post_id='5'  
ORDER BY 
	P.post_dt DESC