Apex domains cannot have CNAME records.
Question and response
Question
dig gda.pl. NS
Answer
gda.pl. 5 NS ns.tpnet.pl.
gda.pl. 5 NS ns1.task.gda.pl.
gda.pl. 5 NS ns3.task.gda.pl.
gda.pl. 5 NS ns2.task.gda.pl.
gda.pl. 5 NS bilbo.nask.org.pl.
Authority
The response has no authority records.
Additional
ns1.task.gda.pl. 5 A 153.19.250.101
ns2.task.gda.pl. 5 A 212.122.192.44
ns3.task.gda.pl. 5 A 213.192.67.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0