I have had it happen on several occasions where I try to update software and I get a GPG error like this:
$ sudo apt-get update W: GPG error: http://inverse.ca lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 19CDA6A9810273C4
If you ever run into an issue similar to this when running apt-get update, take the GPG key fingerprint (in the above example the fingerprint is 19CDA6A9810273C4) and attempt to retrieve it from the Ubuntu keyserver, then add it to your aptitude keys:
$ gpg --keyserver keyserver.ubuntu.com --recv 19CDA6A9810273C4 gpg: requesting key 810273C4 from hkp server keyserver.ubuntu.com gpg: key 810273C4: public key "Inverse Support (package signing) <firstname.lastname@example.org>" imported gpg: Total number processed: 1 gpg: imported: 1
So, the key was found, now add it to our aptitude keys:
$ gpg --export --armor 19CDA6A9810273C4 | sudo apt-key add -
Now, when you run apt-get update you should have no errors. Thank you Artur Iralin for the great post on blogspot.