So I have GCP set up and Kubernetes, I have a web app (Apache OFBiz) running on pods in the GKE cluster. We have a domain that points itself to the web app, so essentially it's accessible from anywhere on the internet. Our issue is since this is a school project, we want to limit the access to the web app to the internal network on GCP, we want to simulate a VPN connection. I have a VPN gateway set up, but I have no idea what to do on any random computer to simulate a connection to the internal network on GCP. Do I need something else to make this work? What are the steps on the host to connect to GCP? And finally, how do I go about limiting access to the webapp so only people in the internal network have access to the webapp?
To setup a VPN on GCP you can use Cloud VPN using static or dynamic routing, you will need to configure a remote peer from the location you want to access your GCP resources to establish the connection towards the Cloud VPN gateway on GCP end.
This means you may require a router that supports creating VPN tunnels on your on-premises or use a host that acts like a router to establish this connection using a VPN software towards Cloud VPN (like Strongswan, for example).
You can block external access to the resources on your VPC network by using GCP firewall rules and just allow specific ports or source IP ranges as you wish.
Another option, even if it's not a VPN or encrypted traffic, is to only allow ingress traffic from the public IP from where you would like to connect to your internal VPC, but this is less secure and would only work if you have an static public IP on your on-premises.