Apex domains cannot have CNAME records.
Question and response
Question
dig d.gcdn.co. NS
Answer
d.gcdn.co. 4 NS gns1.gcdn.co.
d.gcdn.co. 4 NS gns2.gcdn.co.
Authority
The response has no authority records.
Additional
gns1.gcdn.co. 4 A 92.223.100.101
gns2.gcdn.co. 4 A 92.223.77.101
gns1.gcdn.co. 4 AAAA 2a03:90c0:9990:0:0:0:0:1000
gns2.gcdn.co. 4 AAAA 2a03:90c0:9990:0:0:0:0:2000
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0