If you are deploying a custom Remote Desktop Services Farm in Azure, you need to take a look to the Network Card of the VM that acts as a RDP Gateway for it. If you have multiple RDP Gateways behind a load balancer, you would need to use the Public IP of the Load Balancer itself.
If you are deploying a Windows Virtual Desktop host pool in Azure, the gateway is managed by azure and there is not a public registry of the IPs it uses. You can check the URLs here: https://learn.microsoft.com/en-us/azure/virtual-desktop/overview#requirements