Overlapping IP with GWLB

You can connect VPCs with the same (or overlapping) IP ranges to Transit Gateway but you cannot route between them. Transit Gateway doesn't provide any functionality to NAT from one VPC to another with the same range.

This also means that if you have VPC A and B with the same range and VPC C with a different range: If they're all connected to Transit Gateway then A can communicate with C; or B can communicate with C but not both at the same time; nor can A communicate with B - because they have the same IP range.

You could (in some circumstances) use NAT Gateway to provide functionality.

What I think you're actually asking is: If you're using the centralised inspection pattern (where inspection is performed in a VPC connected to Transit Gateway) how does the overlapping IP range work? Answer: It doesn't - for the reasons above. You would need to inspect in each VPC separately using Gateway Load Balancer endpoints in each VPC.

Aviatrix is an AWS partner and has a solution for the Overlapping IP challenge. Take a look at this video

https://youtu.be/KSrt3htsGkA

You should reach out to info@aviatrix.com https://aviatrix.com/learn-center/cloud-networking/handling-overlapping-ips/