After a migration, some external users might lose access to your sites at the destination.
External users must be available in your Active Directory (AD) or MS Entra ID for ShareGate Migrate to map them.
If your external users are part of your user information list in SharePoint and unavailable in your AD or MS Entra ID, you cannot map them correctly in most cases.
As long as you can resolve external users in your SharePoint site, ShareGate Migrate will map them and allow you to change their mappings.
If you cannot resolve these users, you will have to manually re-share the content with them so that they can access it at the destination.