@Nagashree Balasundaram Based on my understanding, you have configured multiple custom domains for Gateway but in the portal, Gateway URL was shown "devpoc-api.*.com" and would like to understand why.
By default, it points to
https://<apim-name>.azure-api.net. After you set up custom domains, it will sort based on certificate, defaultSSLBinding in HostnameConfiguration if available for HostnameType
DeveloperPortal, or default to first custom domain. This value cannot be changed and normally this will not affect any use scenario.
If you are concerned about default certificates issues for Gateway, check out How API Management proxy server responds with SSL certificates in the TLS handshake which describes more on that topic.
For any other questions, feel free to reach out. Would be happy to answer any.