By default, the gateway uses direct TCP network communication. However, as shown in the following image from the On-premises data gateway application, the gateway can be forced to exclusively use HTTPS via the Network tab: 

In the preceding example, a user has opened the gateway application from the server on which a gateway instance has been installed. Unlike modifying the service account per the previous section, however, the user does not need to sign in to Power BI from the gateway application to enable the HTTPS mode. A restart of the gateway is required to apply the change to the HTTPS mode and thus this modification should only be implemented when minimal or no query requests are being processed.

Once the HTTPS mode has been applied, the gateway will strictly use FQDN only and no communication will happen using IP addresses. As advised in the gateway application, enabling the HTTPS mode may slow the performance of gateway requests.