Apex domains cannot have CNAME records.
Question and response
Question
dig gdnsdef.com. NS
Answer
gdnsdef.com. 5 NS sodium.gdnsdef.com.
gdnsdef.com. 5 NS zinc.gdnsdef.com.
Authority
The response has no authority records.
Additional
zinc.gdnsdef.com. 5 A 45.116.42.1
sodium.gdnsdef.com. 5 A 45.116.42.2
zinc.gdnsdef.com. 5 AAAA 2404:ae00:3266:0:0:0:2d74:2a00
sodium.gdnsdef.com. 5 AAAA 2404:ae00:3266:0:0:0:2d74:2a01
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0