Off hand, it looks like it should be $GAB_ads[1];
Also, there are no $GAB_ads[1] in any of your templates.
PHP Code:
<?php
global $GAB_ads;
$ouput .= $GAB_ads[1]; //replace the 0 with whatever your config is.
?>
If you put that in a file and save it as
gab
.php into geek/gars/mods/display/ you can register it right? And it doesnt work in any of your templates? Can you double check the file and make sure you didnt save it with extra text formating or anything?