Thank you for reaching out.
I understand you wish to know more IP address availability of GatewaySubnet.
Yes as mentioned by you above GatewaySubnet is managed by Azure. The IP addresses in the gateway subnet are allocated to the gateway VMs and gateway services. You should never deploy any other service in GatewaySubnet. This is currently documented here. The backend IP allocation of this subnet is not shared with the customers as it is internal to Azure.
Although if it helps you can use this Rest API to check if any particular IP is available in this subnet. As this subnet is managed by Azure the Ip allocation is not promised to remain same always.
Sample Screenshot of the Rest API for an IP in use in the subnet.
Hope this helps! Please let me know if you have any questions. Thank you!
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