I was thinking of using a bionic rule to merge the manager's GSID with the CSM's GSID... is there a better way?
Best answer by kunal_bhat
You're getting that error since the Manager field is already mapped in the direct mappings section, there's no more GSID fields left to add.
You ned to unmap the Manager field from the Direct Mappings section which will allow you to add the Derived Mapping for Manager.
To unmap the Manager field you need to set the source field as "None" as below.
Once you've done that, you can configure your import lookup for Manager pretty much like what Kate has done.