SharePoint allows you to share content with people outside of your organization. When you do so, they will receive an external sharing link.
ShareGate Desktop does not support the preservation of external sharing links during a migration.
When you migrate an environment with external sharing links, you are changing the location of these links. Because of this, a new invitation needs to go out to the users with permission to view the content through these links.
You must manually reshare the links in the destination of your migration.