First off GPG stands for GNU Privacy Guard , and is a open source implementation of PGP.
So what can you do with GPG , and why should we care?
One common use of is to sign your messages, this way the receiver can verify it did in fact come from you, and that it hasn’t been altered.
-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: SKS 1.1.4
Comment: Hostname: keyserver.ubuntu.com
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=pePB
-----END PGP PUBLIC KEY BLOCK-----
Using the above public key we can verify a signed message I wrote.
For example below is a message I signed with the text “Here is a signed message from me!" .
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512
Here is a signed message from me!
-----BEGIN PGP SIGNATURE-----
Version: GnuPG/MacGPG2 v2.0.22 (Darwin)
Comment: GPGTools - https://gpgtools.org
iQEcBAEBCgAGBQJS67emAAoJEBWBnFv4DSwoFmAH/2FNYG8JODEYvWna3kGYDHZ3
I1Td5fGxLc2vcpiRyiZJ9RATByV6sact+ITHduj7TPTDo/zrDDnoqDnAt59kZhAn
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QMQ4bxG3z+Pz3lbY5FuW97GtFgpe1BEVBxcDZU92njO6mAK4PJn6M5B8dMP4Z1I=
=/p36
-----END PGP SIGNATURE-----
To verify this message we can use GPG’s –verify option:
$ gpg --verify
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512
Here is a signed message from me!
-----BEGIN PGP SIGNATURE-----
Version: GnuPG/MacGPG2 v2.0.22 (Darwin)
Comment: GPGTools - https://gpgtools.org
iQEcBAEBCgAGBQJS67emAAoJEBWBnFv4DSwoFmAH/2FNYG8JODEYvWna3kGYDHZ3
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QMQ4bxG3z+Pz3lbY5FuW97GtFgpe1BEVBxcDZU92njO6mAK4PJn6M5B8dMP4Z1I=
=/p36
-----END PGP SIGNATURE-----
gpg: Signature made Fri Jan 31 08:48:06 2014 CST using RSA key ID F80D2C28
gpg: Good signature from "nessy <..@gmail.com>"
gpg: aka "nessy <..@flip-edesign.com>"
As seen above the, the signature states Good signature , and shows when it was signed.