Private Connectivity Options: AWS Commercial VPC to GovCloud VPC

GovCloud has physical and logical (network) isolation from the rest of the regions for compliance reasons (mentioned here), so native solutions like VPC peering or TGW attachments aren't available. You can implement S2S VPN over the Internet to establish connectivity.

Another option might be to build separate private VIFs/VPN tunnels to a VPC in each region (GovCloud and non-GovCloud) and then route traffic via the on-premises network, but that may not be ideal from a latency perspective.