AWS Transit Gateway provides statistics and logs that are then used by services such as Amazon CloudWatch and Amazon VPC Flow Logs. You can use Amazon CloudWatch to get bandwidth usage between Amazon VPCs and a VPN connection, packet flow count, and packet drop count. You can also enable Amazon VPC Flow Logs on AWS Transit Gateway so you can capture information on the IP traffic routed through the AWS Transit Gateway.
AWS Transit Gateway Network Manager includes events and metrics to monitor the quality of your global network, both in AWS and on premises. Event alerts specify changes in the topology, routing, and connection status. Usage metrics provide information on up/down connection, bytes in/out, packets in/out, and packets dropped.