Firewall rules in GCP let you allow or deny traffic based on rules you apply to your instances. Firewall rules are applied at a networking level with every VPC network acting as a distributed firewall. Even though firewall rules are applied at a networking level, they allow/deny connections at an instance level. The advantage of a distributed firewall is that it does not only filter traffic between instances but also filters traffic between networks. All firewall rules are specific to a VPC network with each rule either allowing or denying traffic.
Firewall rules, being unique to a VPC network, cannot be shared between multiple VPC networks. It is important to note that firewall rules in GCP only support IPv4 traffic. There is also ...
