With GnuPG you can sign messages. The signature looks something below. Does this contain more than just a hash, and if so, what/how can this be made visible?
OpenPGP, which GPG implements, is a public-private key scheme. That is, it is founded on a two-part key, one of which is made publicly available and the other kept only by the owner. Messages encrypted with one of the two keys can be decrypted with the other. To send a secret message to someone, you would encrypt the message with their public key, and only they would be able to decrypt it with their private key. To digitally sign a message and demonstrate that you were the originator, you make a hash and encrypt that with your private key, and everyone would be able to decrypt it with your public key and confirm that it had to have been made with access to your private key (i.e. by you, unless something has gone terribly wrong).