My configuration
1. Created a VPC (poller-vpc)
2. Created two subnets (private subnet[0.0.1.0/27] , public subnet[0.0.1.32/27])
3. Added firewall rule (tcp, udp, icmp for 0.0.0.0/0 for vpc-poller network)
4. Created "nat-instance" in public subnet having both private/ public IP, can forward ip and tag = "nat"
5. Created "test-instance" in private subnet having only private IP, having tag = "no-ip"
6. Created route for destination (0.0.0.0/0) tags = "no-ip" and next-hop-instance = "nat-instance"
7. Logged in to "nat-instance" and ran ["sudo sysctl -w net.ipv4.ip_forward=1", "sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE"]
8. Then ssh into private instance and tried apt update but not able to connect to internet from private instance through nat.
I followed cloud.google.com/vpc/docs/special-configurations
Thanks Ayush