Hi @Larry2,
I haven't worked on a GCC environment before so I won't be able to provide much insight specificly around it. That said, the documentation does mention for US Gov environments to enable tables for search in the classic solution explorer:
If you are selecting tables for Dataverse search on a Power Apps US Government environment, use the legacy solution explorer to choose the tables to be indexed for Dataverse search.
Have you tried doing it via this method? If that doesn't help, I'd suggest raising a ticket with Microsoft support since they advised you that it was safe to use it in prod even though it's still preview in GCC.
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