These are instructions for installing OpenVPN. Use algo to setup ipsec & WireGuard instead. Algo supports GCE among other providers. By using ipsec, you don't need to install anything extra on most clients. By using WireGuard, you have a state-of-the-art VPN.

https://github.com/trailofbits/algo

Only problem I see here is that the free tier of GCP limits you to "1 GB network egress" per month. How long before you run through that? (Not long, I think.)

https://cloud.google.com/free/