GnuPG is a publicly available encryption scheme that uses the "public key" approach. With GnuPG, messages are encrypted using a "public key" however, they can only be decrypted by a "private key", which is retained by the intended recipient of the message.
You can also import an existing key by clicking the "Import Key" button at the bottom of the same page. Then simply copy the key code you want, and paste it into the text area provided. Click the "Import" button, and the key will be added to your account.