_spf1.gtnexus.com has no CNAME records.
Question and response
Question
dig _spf1.gtnexus.com. NS
Answer
The response has no answer records.
Authority
gtnexus.com. 37525 SOA edns2.ultradns.com. premiumdns.support.neustar. 2019053537 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0