Select the Character gameObject in the GKC_Prefab.
Go to Inventory List Manager inspector and open the inventory list.
Press the button Add Object.
Open the new object and configure the Name and description.
Then, create the mesh used for that inventory object. It needs to have these settings:
The mesh or meshes used in the object are placed in an empty parent, with a scale (1,1,1).
That parent needs to use the layer inventory.
That parent needs a collider adjusted to the object's shape and size. You can add as many collider (box, sphere, capsule) as you need if the objects has a complicated shape, but all these colliders need to be configured in the parent.
This is not necessary, but the name for the prefab can be the name of the object with Mesh and the end.
For example, this is the red wire created.
Once it is created, drop it to the prefab folder in the path Assets/Game Kit Controller/Prefabs/Inventory/Mesh.
Go back to the Inventory List Manager, and set the mesh of the new object with the one created previously in the field Inventory Object Mesh.