Sorry but today it is not possible to change the Gateway FQDN after initial setup. Only way for you to start using an externally resolvable FQDN would be for you to redeploy the whole vApp.
All access must be made to the one and the same FQDN. So your internal users will use the same FQDN. Most implementations use Split DNS so your internal users resolve the FQDN to internal IP rather than the external one.