GCP charging for interzone traffic is an interesting financial choice. They own all the infra and in many cases this is literally moving from building to building.
There's cross-region, and cross-zone. If both boxes are located within the same zone (e.g. us-east1) then the bandwidth is free, since it's intrazone traffic. Cross-zone egress traffic (e.g. us-east1 to us-central1) is billed at a certain rate, and cross-region egress traffic (e.g. us-east1 to europe-west8) is billed at a significantly higher rate.
Amusingly enough, ingress traffic seems to always be free. So you can upload as much data as you want into their cloud, but good luck if you need to get it out.
I am referring to cross-zone within in the same region, so like us-central1-a to us-central1-b. These are building to building and often never cross public land.
Amusingly enough, ingress traffic seems to always be free. So you can upload as much data as you want into their cloud, but good luck if you need to get it out.