I initially posted this question in the Joomla forum without realizing it - but I was wondering how to use different backgrounds on different pages of a Gantry WordPress site. Well I kept messing around and found the answer:
Created an override then went to Gizmos and checked Page Class Suffix, then added the suffix.
In my custom style sheet (which is a custom sheet added to the index.php css array) I added the style for the body.myclass {background-image: etc., etc.,