Running http://whois.nic.$GTLD/ is required, but I believe it's fine to have nothing on nic.$GTLD directly.
Some examples: https://whois.nic.sharp http://whois.nic.amazon https://whois.nic.horse https://whois.nic.google https://whois.nic.trust
See the gTLD guidebook here: https://newgtlds.icann.org/sites/default/files/guidebook-ful...
Section 4.1 on page 268 of the PDF covers whois.nic.$GTLD