NAT issues on Google Cloud – Server error

I've created a NAT Cloud according to the googles reference implementation.

After creating a NAT, I see a route to IGW in my VPC ("Default Route to Internet" with a range of IP addresses of Desitnation and a Next to to Default Internet Gateway). I was expecting Internet traffic to be routed to NAT instead of IGW. (I started with AWS where IGW and NAT are separated). How does the traffic cross NAT if the road points to IGW?

Moreover, when I try to determine my IP using "curl", I get I was waiting to get the IP address of the NAT service. obviously does not work for the IP whitelist. What is missing in my configuration to recover NAT IP address when curling my IP address?