First, Gantry doesn’t use wp_nav_menu() as is the WordPress standard, so you’ll need to use UberMenu Easy Integration. The easiest solution is to place a text widget in the menu widget area and set the widget’s content to [uberMenu_easyIntegrate]. Don’t forget to follow the remainder of the UberMenu Easy Integration steps.

Second,
(this step should no longer be required with current versions of UberMenu
Gantry’s custom menu item options override UberMenu’s. To fix this, you may need need to edit core/UberMenu.class.php line 423:

add_filter( 'wp_edit_nav_menu_walker', array( $this , 'editWalker' ) , 2000);

If the UberMenu options don’t appear, increase 2000 to 5000.

Third,
(this step is no longer required with versions of Gantry released after 10 October 2013)
Gantry seems to have a bug regarding widget areas, so if you place a widget in UberMenu, you’ll get an error. To solve this, add a check to the Gantry function to ignore widgets without their custom widget_map parameter:

if( !isset( $params[0]['widget_map'] )) return $params;

This would be placed in plugins/gantry/core/renderers/gantrywidgetsrenderer.class.php, around line 148, as the first line in the filterWidget() function. I have detailed this issue here, and I hope it will be resolved by Gantry in their next version.