Hi @Kim Williams,
Unfortunately, since the ShareGate you are using is a third-party tool, we lack information about it. We are afraid that we cannot provide you with any further information on how to solve your problem.
But here I am offering you another way to copy your sites to another server, hopefully this method will be successful in copying your sites:
SharePoint on-premises – migrate site collection to new server
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make sure that you completely understand the risk before retrieving any suggestions from the above link.
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