I setup a brand new website running GL Version 2.1.1 and among other things installed GUS Version 1.7.3.
The stats collection was working just fine but every time I would try to access the GUS Admin interface I would be greeted with the following error:
Fatal error: Call to undefined function: ADMIN_createMenu() in /home/PATHTO/admin/plugins/gus/index.php on line 651
I searched high and low to discover what might be going on here and was in the dark until I found a post answered by Dirk:
Fatal error using admin functions
The solution was found in the reply:
"Make sure you have a file system/lib-admin.php (that one was introduced in 1.4.0, actually)."
I did a check and found I did have that library so I added the following to my
require_once $_CONF['path_system'] . 'lib-admin.php';
and bye-bye error... hello GUS Admin - somewhere along the line the library was not getting included. There may be a more appropriate time to call lib-admin.php but this addition to the GUS Admin index.php file solved my problem - yours too I hope!