Importing/Adding GPG keys to Apt-Get/Aptitude

If you get messages like this...
W: GPG error: http://dl.google.com stable Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A040830F7FAC5991
...when you are updating your package list via apt-get or aptitude, you have to import the missing GPG key, in order to avoid these warning messages in the future:
gpg --recv A040830F7FAC5991
gpg --export --armor A040830F7FAC5991 | sudo apt-key add -
In this way you can add a new GPG signature key for the distributed packages to apt-get.