I created two dns zones on Google Cloud Platform with the same "A"
type pointing out to the same domain. Now my site returns Error 522.
This error is not related to having two DNS zones. You can have as many zones as you want, only the zone that serves as your domain's name servers is in effect. However, except for backup reasons, there is no reason to have two zones providing the same DNS resource records.
HTTP Error 522 means that a TCP connection to your site is timing out (your site is not responding). Go to your domain registrar. Verify that the DNS name servers listed are in fact in your DNS Zone (check each dns server name entry). If each entry is correct and point to a name server record in your zone, check the A resource record. Make sure the A record points to the correct IP address for your website. If that is correct, then the problem is a firewall rule blocking access, an incorrectly configured web server or other software problem. Repease for the www DNS resource record entry.
Documentation says it cannot be deleted. Does that mean that I
nullified my domain?
The reason you can not delete the zone is that the zone is not empty. The zone has resource records (A record, CNAME record, etc). Delete each entry one by one and then you can delete the zone.