The plugin provides a class element that you can use in your theme, but you must instantiate it first:
`$myMobile = new Mobile();`
then you can use
`$myMobile->isMobile()` to test if you are running on a mobile device.
But the primary purpose of this theme is to automatically switch to a mobile theme when the client is browsing from a mobile device. If all you want to do is use the above `isMobile()` method you might be better off loading the underlying class script: Mobile_Detect. The plugin class `mobile` is a descendent from `Mobile_Detect` with additional code for handling the theme switching.
As to your error message, I doubt that is from the plugin (the call is from your theme's `inc_header.php` script.) I have no idea what variable isGallery would be, not something in Zenphoto 1.4.3