Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hi Richard,
Thanks for posting the query here in the community.
Based on your description the Microsoft Commercial O365 Cloud and Microsoft Government O365 Cloud are different Office 365 tenant, you may try to migrate mailboxes and service settings from one Office 365 tenant to another Office 365 tenant in a business-merger scenario. The detailed steps you can refer in this link:How to migrate mailboxes from one Office 365 tenant to another.
Here are compliance between Microsoft 365 Commercial and GCC for your reference.
I also found one thread you may refer this thread. In this thread mention how to migrate Office 365 from one tenant to another tenant.
Please let us know if you need further help.
Regards,
Waqas Muhammad