Then dget fails to verify the source, because it is missing a public key.

Dpkg::Control::Fields::field_capitalize() called too early to check prototype at /usr/share/perl5/Dpkg/Control/Fields.pm line 277.
Dpkg::Control::Fields::field_capitalize() called too early to check prototype at /usr/share/perl5/Dpkg/Control/Fields.pm line 278.
gpgv: Signature made 2017年08月11日 星期五 02时51分20秒 CST using RSA key ID 864CC8BF
gpgv: Can't check signature: public key not found
dpkg-source: warning: failed to verify signature on ./gflags_2.2.1-1.dsc

@-OptiPlex-790:/$ sudo gpg --keyserver keyserver.ubuntu.com --recv-key 864CC8BF
gpg: requesting key 864CC8BF from hkp server keyserver.ubuntu.com
gpg: key B933BBBB: "Laszlo Boszormenyi (GCS) <gcs@gcs.org.hu>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

@-OptiPlex-790:/$ sudo gpg --fingerprint 864CC8BF
pub   4096R/B933BBBB 2012-09-01
      Key fingerprint = A0DF 7E0D 3851 E0EE 45C0  0BC8 ACE1 F33C B933 BBBB
uid                  Laszlo Boszormenyi (GCS) <gcs@gcs.org.hu>
uid                  Laszlo Boszormenyi (GCS) <gcs@lsc.hu>
uid                  Laszlo Boszormenyi (GCS) <gcs@c64.org>
uid                  Laszlo Boszormenyi (GCS) <gcs@debian.hu>
uid                  Laszlo Boszormenyi (GCS) <gcs@debian.org>
sub   4096R/864CC8BF 2012-09-01

DSCVERIFY_KEYRINGS="/etc/apt/trusted.gpg:~/.gnupg/pubring.gpg"

DSCVERIFY_KEYRINGS="~/.gnupg/pubring.gpg:/etc/apt/trusted.gpg"