Resolution Key Configuration
Admins can select unique identifiers known as Resolution keys for person records from the fields of the Person object. These keys are used to identify unique Person records in Gainsight, while inserting new records or updating existing records in the Person object, through any data ingestion channel in Gainsight. These keys are honored across all of the Gainsight data ingestion channels, such as Salesforce Connector, Bulk Person upload through CSV, uploading People through Rules, etc.
For more information, refer to the Resolution Key Configuration in the Additional Resources section at the end of this article.

Users cannot add a person unless they provide the Resolution key while adding a person. For example, in the Add Person dialog box, the Email field becomes a mandatory field if email has been set as a resolution key. Leaving the field blank will not let the users add the person.
IMPORTANT: Whenever users add a person from anywhere in the application, this rule is applicable. Without providing the mandatory information that is set as the resolution key, users cannot add a person.