Apex domains cannot have CNAME records.
Question and response
Question
dig gtt-net.cz. NS
Answer
gtt-net.cz. 5 NS ns1.gtt-net.cz.
gtt-net.cz. 5 NS ns2.gtt-net.cz.
Authority
The response has no authority records.
Additional
ns1.gtt-net.cz. 5 A 82.144.129.1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0