How to link a base unit with an existing case unit in GPE?

To add an upper level to an item, of this upper level already exists in the system, then locate the upper level GTIN and click 'Edit'. This GTIN will no have a status of pending. 

Go to the Packaging and Hierarchy menu. 

In this menu you can simply add new child trade item, and complete the child quantity

Don't forget to save, before leaving the page 

Now simply complete any further actions before validation and publishing your product/s