[BirdOPrey5]

This is the code that creates $gthreads, which each individual thread becomes $gthread:

$gthreads = $db->query_read(" 
    SELECT 
thread.forumid AS forumid, thread.visible,
	thread.lastposter, thread.prefixid, thread.title, thread.threadid as threadid, thread.attach AS attach, 
	thread.taglist AS tag, thread.deletedcount AS deletedcount, thread.postusername, thread.postuserid, thread.dateline AS date, views AS views, thread.replycount AS reply, thread.lastposter, 
	forum.title_clean as forum_title, id, post.pagetext AS pagetext, post.postid, post.pagetext AS pagetext,
	user.userid, user.username, user.usergroupid, icon.title AS icontitle, icon.iconpath,
	IF(displaygroupid=0, user.usergroupid, displaygroupid) AS displaygroupid, infractiongroupid
	FROM " . TABLE_PREFIX . "thread AS thread
	LEFT JOIN " . TABLE_PREFIX . "post AS post on (thread.firstpostid = post.postid)
	LEFT JOIN " . TABLE_PREFIX . "forum AS forum on (thread.forumid = forum.forumid)
  LEFT JOIN " . TABLE_PREFIX . "user AS user ON(user.userid = thread.postuserid)
  LEFT JOIN " . TABLE_PREFIX . "icon AS icon ON(icon.iconid = thread.iconid)
        WHERE NOT ISNULL(thread.threadid) 
        $excludedforums AND thread.visible = 1 ORDER BY postid DESC LIMIT 0, $dzmmax");

The only usernames options for $gthread are:

$gthread[lastposter],
$gthread[postusername]

If it's not one of those it's not available.