@Keith Oswalt @Anonymous It seems that you are using a different account in this community, just a friendly reminder, please use the same account that you posted the question to follow up your thread. :)
For your issue, it looks like about the 3rd party migration tool(ShareGate), we suggest you contact ShareGate support for better help. Thanks for your understanding.
Microsoft provides third-party contact information to help you find technical support. This contact information may change without notice. Microsoft does not guarantee the accuracy of this third-party contact information.
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