GRE Tunnel with TGW Use Cases

Hello.

The following documentation may be helpful.
https://aws.amazon.com/jp/blogs/networking-and-content-delivery/simplify-sd-wan-connectivity-with-aws-transit-gateway-connect/

Connecting on-premises networks to AWS - GRE tunnels can be used to connect VPN appliances or routers on-prem to TGW, providing secure connectivity to VPCs and on-prem networks. This avoids the need for each VPC having its own VPN connection.

Traffic inspection/security services - The GRE tunnels can send traffic to a firewall or IPS appliance before routing it to the destination VPC. This allows you to apply security policies and inspect traffic.

Another use case that I've personally deployed for some customers is to connect the VPC (or VPCs) to different on-premises VRF networks. This is the case mainly for CSPs or Telcos which have different routing-domains or VRFs on-premises and need to connect these networks to AWS, keeping this separation of routing. This is usually done by using TGW Connect peers (GRE tunnels) or Private IP IPSec VPNs, which both terminate on TGW. From here, you can create multiple TGW route-tables to keep the routing separated/segregated, similar to how it's done on-premises with VRFs.