Hi @sakuraime ,
The flexible server architecture allows users to opt for high availability within single availability zone and across multiple availability zones depending upon which option we select. If we go through the diagrams provided in the article, we would find either of them be applicable at a time. So in case of zone redundancy if failure happens it will switch on to replica from another availability zone.
Gateway ip address is common for Azure MySQL as it uses the same firewall engine. I would suggest you to please go through the below link to understand the networking aspect of Flexible Server:
https://video2.skills-academy.com/en-us/azure/mysql/flexible-server/concepts-networking