After adding a new apt repository to your sources.list you often get a gpg error that there is no public key for that repository. If you trust the added server you can add the key with the command stated below.
Here is an example error message. I replaced the id of the key with <long_key>:
“GPG error: http://archive.ubuntu.com oneiric Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY <long_key>”
And here is how to import the key:
gpg --keyserver keyserver.ubuntu.com --recv-keys <long_key>
gpg --export --armor <long_key> | sudo apt-key add -