Kubernetes - Pass Public IP of Load Balance as Environment Variable into Pod

I know this isn't the exact approach you were going for, but I've found that creating a static IP address and explicitly passing it in tends to be easier to work with.

First, create a static IP address:

gcloud compute addresses create gke-ip --region <region>

where region is the GCP region your GKE cluster is located in.

Then you can get your new IP address with:

gcloud compute addresses describe gke-ip --region <region>

Now you can add your static IP address to your service by specifying an explicit loadBalancerIP.¹

apiVersion: v1
kind: Service
metadata:
  name: gateway
spec:
  selector:
    app: gateway
  ports:
    - name: http
      port: 3000
      targetPort: 3000
      nodePort: 30000
  type: LoadBalancer
  loadBalancerIP: "1.2.3.4"

At this point, you can also hard-code it into your ConfigMap and not worry about grabbing the value from the cluster itself.

¹If you've already created a LoadBalancer with an auto-assigned IP address, setting an IP address won't change the IP of the underlying GCP load balancer. Instead, you should delete the LoadBalancer service in your cluster, wait ~15 minutes for the underlying GCP resources to get cleaned up, and then recreate the LoadBalancer with the explicit IP address.

You are trying to access gateway service from client's browser.

I would like to suggest you another solution that is slightly different from what you are currently trying to achieve but it can solve your problem.

From your question I was able to deduce that your web app and gateway app are on the same cluster.

In my solution you dont need a service of type LoadBalancer and basic Ingress is enough to make it work.

You only need to create a Service object (notice that option type: LoadBalancer is now gone)

apiVersion: v1
kind: Service
metadata:
name: gateway
spec:
selector:
  app: gateway
ports:
  - name: http
    port: 3000
    targetPort: 3000
    nodePort: 30000

and you alse need an ingress object (remember that na Ingress Controller needs to be deployed to cluster in order to make it work) like one below: More on how to deploy Nginx Ingress controller you can finde here and if you are already using one (maybe different one) then you can skip this step.

apiVersion: networking.k8s.io/v1beta1
kind: Ingress
metadata:
name: gateway-ingress
annotations:
  nginx.ingress.kubernetes.io/rewrite-target: /
spec:
rules:
  - host: gateway.foo.bar.com
    http:
      paths:
      - path: /
          backend:
            serviceName: gateway
            servicePort: 3000

Notice the host field.

The same you need to repeat for your web application. Remember to use appropriate host name (DNS name) e.g. for web app: foo.bar.com and for gateway: gateway.foo.bar.com and then just use the gateway.foo.bar.com dns name to connect to the gateway app from clients web browser.

You also need to create a dns entry that points *.foo.bar.com to Ingress's public ip address as Ingress controller will create its own load balancer.

The flow of traffic would be like below:

+-------------+   +---------+   +-----------------+   +---------------------+
| Web Browser |-->| Ingress |-->| gateway Service |-->| gateway application |
+-------------+   +---------+   +-----------------+   +---------------------+

This approach is better becaues it won't cause issues with Cross-Origin Resource Sharing (CORS) in clients browser.

Examples of Ingress and Service manifests I took from official kubernetes documentation and modified slightly.

More on Ingress you can find here and on Services here