More Information
You can combine gifts with items or other gifts with an anvil, even if it doesn't show anything in the result slot, click on it and the anvil will close and drop the combined item.
The 'Capacity' value on the config determines the maximum number of items you can combine using anvils. It does not apply to /gifts edit as that command was intented as an admin tool and not a command normal players should have access to.
You can mine gifts with a silk touch enchanted tool by default (you can disable it in the config).