ShareGate Migrate can crawl through your selected target to list all your external users. You can take action on these users once the report is finished running.
Prerequisites
You have site collection admin permissions on your targeted site collection(s).
Note: Site collection admin permissions are required even if you have higher admin privileges like SharePoint admin or Global admin permissions.
For more information see Microsoft administrative permissions.
Tip: To manage all your site collections, you can connect to your central admin or admin center with SharePoint admin, Farm admin or Global admin permissions.
How-To
To learn how to modify a report and save it (add filters and add/remove columns), see Custom reports.
For more on reports, see Reporting options.