Established connection to StrongSwan VPN running on Ubuntu but can't connect to the internet

Solution 1:

From the GCP firewall side your configuration looks OK. However installing & configuring StrongSwan is not a simple process and there are many steps that determine wheather it's sucessfull or not.

You can try to repeat the process on another VM (created one from scratch) and go over the steps again, but...

If you can use other solutions I'd recommend going for a Marketplace one - deploying them is much simpler and you're getting working solution out of the box - such as OpenVPN. And it's certified to work with GCP.

You can also try SoftEther VPN but there's no Marketplace ready to deploy solution yet for that so it would mean going over the installation as with the StronSwan.