gmail-smtp-in.l.google.com has no NS records.
NS records for this zone are defined at l.google.com.
NS records for this zone are defined at l.google.com.
Question and response
Question
dig gmail-smtp-in.l.google.com. NS
Answer
The response has no answer records.
Authority
l.google.com. 60 SOA ns1.google.com. dns-admin.google.com. 877860005 900 900 1800 60
Additional
. 0 OPT ; payload 512, xrcode 0, version 0, flags 0