Detecting VPC to VPC Traffic Flow via DX Gateway

Per the FAQ:

Q: Can I send traffic from a VPC that is associated with an AWS Direct Connect gateway to another VPC associated to the same AWS Direct Connect gateway?

No, AWS Direct Connect gateway's only support routing traffic from AWS Direct Connect VIFs to VGW (associated with VPC). In order to send traffic between two VPCs, you must configure a VPC peering connection.