I have installed strongswan VPN on GCP Compute engine(Ubuntu 20.04) by referring https://www.digitalocean.com/community/tutorials/how-to-set-up-an-ikev2-vpn-server-with-strongswan-on-ubuntu-20-04
have 3 different projects and I set up a tunnel for all from Strongswan VPN Compute Engine.
below is the ipsec.conf file
conn strongswan-to-ops
ikelifetime=600m # 36,000 s
keylife=180m # 10,800 s
rekeymargin=3m
keyingtries=3
keyexchange=ikev2
mobike=no
ike=chacha20poly1305-sha512-curve25519-prfsha512,aes256gcm16-sha384-prfsha384-ecp384,aes256-sha1-modp1024,aes128-sha1-modp1024,3des-sha1-modp1024!
esp=chacha20poly1305-sha512,aes256gcm16-ecp384,aes256-sha256,aes256-sha1,3des-sha1!
authby=psk
left=172.x.x.x # In case of NAT set to internal IP, e.x. 10.164.0.6
leftid=172.x.x.x
leftsubnet=192.x.x.x/24
leftauth=psk
right=[gateway IP]
rightid=[gateway IP]
rightsubnet=172.x.x.x/16
rightauth=psk
type=tunnel
auto=start
dpdaction=restart
the IPsec status shows
Security Associations (2 up, 0 connecting):
strongswan-to-ops[4]: ESTABLISHED 46 minutes ago,
strongswan-to-ops{3}: INSTALLED, TUNNEL, reqid 2, ESP in UDP SPIs: c233fb9c_i 17261c13_o
strongswan-to-ops{3}: 192.x.x.x/24 === 172.x.x.x/16
but still not able to access HTTP server/other VM on other projects
On GCP Compute Engine IP Forwarding was off, so that was the problem.
I take a snapshot of the Compute Engine and create new one using it and mark IP forwarding to ON.