Your item metadata is imported automatically during a migration, without the need to map any of the fields in the integration.
For this to happen, you'll need to include a tab in your GatherContent Item, named "metadata". This tab will need to contain fields labelled "meta_description" and "meta_keywords". These fields can be empty if you do not want to import any metadata.
Meta description is mandatory if you want to publish to a Blog Post upon import.