Hello David,
Ingress traffic (e.g into the GCE instance) is free. However Egress traffic (e.g outbound from the GCE instance) depends on the direction. It’s free egress to the same zone (to internal IP), to a different service within the same region, or to some Google products like Youtube. Otherwise you will encounter charges for egress traffic depending on your usage and direction, for further details you may check this document about network pricing. I hope this helps.