"Google Cloud VPC Network Peering allows internal IP address connectivity across two Virtual Private Cloud (VPC) networks regardless of whether they belong to the same project or the same organization."
https://cloud.google.com/vpc/docs/vpc-peering
while
"Cloud Interconnect provides low latency, high availability connections that enable you to reliably transfer data between your on-premises and Google Cloud Virtual Private Cloud (VPC) networks."
https://cloud.google.com/network-connectivity/docs/interconnect/concepts/overview and
"HA VPN is a high-availability (HA) Cloud VPN solution that lets you securely connect your on-premises network to your VPC network through an IPsec VPN connection in a single region."
https://cloud.google.com/network-connectivity/docs/vpn/concepts/overview

The requirement is for a stable set of egress IP addresses from a GKE cluster for allowlisting by a third party, following best practices.
Option A is not recommended: Using a single node lacks scalability and high availability. Relying on a single node's static IP creates a single point of failure and doesn't align with GKE's design principles. Disabling autoscaling hinders elasticity.
Option C is complex and unreliable: Public nodes typically have ephemeral external IPs (unless manually configured per node, which is difficult to manage with autoscaling). Dynamically tracking and emailing IPs daily is operationally burdensome and prone to race conditions where the allowlist might lag behind IP changes.
Option D uses Cloud NAT but with dynamic IPs. Dynamic IPs change over time, making them unsuitable for stable firewall allowlists.
Option B is the Google-recommended practice: Configuring the GKE cluster with private nodes enhances security as nodes don't have direct external IPs. Cloud NAT provides managed network address translation for these private nodes to access the internet. By configuring Cloud NAT with a static allocation of external IP addresses, all egress traffic from the private GKE nodes will appear to originate from this stable, predictable set of IPs. This set can be given to the vendor for allowlisting without worrying about node IP changes due to scaling or maintenance.
This approach decouples the application's egress IP from the individual nodes, providing stability and adhering to the principle of least privilege (private nodes).
References:
Cloud NAT Overview: "Cloud NAT lets certain resources without external IP addresses create outbound connections to the internet." - https://cloud.google.com/nat/docs/overview Cloud NAT IP Addresses: "When you configure a NAT gateway... You can configure the NAT gateway to automatically allocate regional external IP addresses... Alternatively, you can manually assign a fixed number of static external IP addresses to the gateway." - https://cloud.google.com/nat/docs/overview#ip-addresses GKE and Cloud NAT: "Configure Cloud NAT with GKE... Use Case: You want a GKE pod to deterministically egress traffic from a static set of IP addresses that you control." - https://cloud.google.com
/nat/docs/gke-example
Private Clusters: "Private nodes do not have endpoint-accessible external IP addresses." - https://cloud.google.
com/kubernetes-engine/docs/how-to/private-clusters

In general, Google recommends that each instance that needs to call a Google API should run as a service account with the minimum permissions necessary for that instance to do its job. In practice, this means you should configure service accounts for your instances with the following process: Create a new service account rather than using the Compute Engine default service account. Grant IAM roles to that service account for only the resources that it needs. Configure the instance to run as that service account. Grant the instance the
https://www.googleapis.com/auth/cloud-platform scope to allow full access to all Google Cloud APIs, so that the IAM permissions of the instance are completely determined by the IAM roles of the service account.
Avoid granting more access than necessary and regularly check your service account permissions to make sure they are up-to-date.https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for- instances#best_practices Reference: https://cloud.google.com/compute/docs/access/create-enable-service-accounts-for-instances