how to correct GPG error when updating via apt-get update

[Jett Tayer]

I always have this error when updating.

Get:6 http://ftp.easynet.be unreleased/main Packages [241kB]
Fetched 247kB in 15s (16.0kB/s)
Reading package lists... Done
W: GPG error: http://ftp.gnuab.org unstable Release: Unknown error
executing gpgv
W: GPG error: http://ftp.de.debian.org unstable Release: Unknown error
executing gpgv
W: GPG error: http://ftp.gnuab.org unreleased Release: Unknown error
executing gpgv