Hi Mark, apologies for the late reply.
The GMS does not account for existing master library items, therefore, you may see dupes if you already have that same metric in the app when you run GMS. We have it on the roadmap to replace existing master library items that pre-exist.
Yes, the GMS creates a set of tags. The tags are created to help identify GMS applied metrics, make them searchable, and used for internal management in some other solutions we are developing. In addition, when deleting GMS items from an app, the MasterItem tag is used to identify the items for GMS to delete.