If it is a single image, you could edit the GAB_item_image template, adding in HTML to center the image. Edit the GAB_item_rotating template for rotating images. For other types of images, look at "Template for html/text ad" via the ACP ->
GAB
-> Banners -> click a
banner
link and edit that template. Alternatively, put the wrapper back on and then edit the GAB_wrapper_standard template to get the look you want.