In 1991, Phil Zimmerman created an encryption program called Pretty Good Privacy (or, PGP). His creation eventually inspired an encryption standard called OpenPGP. GPG, or GnuPG, is a complete and free implementation of this encryption standard. The official GPG website tells us that

GnuPG allows you to encrypt and sign your data and communications; it features a versatile key management system, along with access modules for all kinds of public key directories. GnuPG, also known as GPG, is a command line tool with features for easy integration with other applications. A wealth of frontend applications and libraries are available. GnuPG also provides support for S/MIME and Secure Shell (ssh).

https://www.gnupg.org/

Here are the basic commands to get started with GPG:

Create new key: gpg --full-generate-key

Delete public key: gpg --delete-keys [name/fingerprint]

Delete private key: gpg --delete-secret-keys [name/fingerprint]

Delete private and public key: gpg --delete-secret-and-public-keys [name/fingerprint]

List private keys: gpg -K

List public keys: gpg -k

Export key to file: gpg --armor --export [name/fingerprint] > [file].asc

Import key from file: gpg --import [file].asc

Encrypt a file: gpg --encrypt --armor --recipient [recipient name/fingerprint] [file]

Decrypt a file: gpg --decrypt [file]

Verify signature: gpg --verify [file].asc

Edit key: gpg --edit-key [name/fingerprint]

Change passphrase: gpg --passwd [name/fingerprint]