GnuPG is a cryptography tool that helps you manage public and private keys as well as perform encrypt, decrypt, sign, and verify operations. It is an open-source version of PGP. This tutorial will go over basic key management, encrypting (symmetrically and asymmetrically), decrypting, signing messages, and verifying signatures with GPG.
Let's Encrypt is "a free, automated, and open Certificate Authority." They provide free signed certificates as a trusted certificate authority. This tutorial walks through the process of installing certbot and requesting new certificates and renewing existing ones wit Let's Encrypt. If you are just looking to generate your own quick self-signed certificates, check out my tutorial on creating self-signed SSL certificates with OpenSSL.
This tutorial will walk through the process of creating your own self-signed certificate. You can use this to secure network communication using the SSL/TLS protocol. For example, to run an HTTPS server. If you don't need self-signed certificates and want trusted signed certificates, check out my LetsEncrypt SSL Tutorial for a walkthrough of how to get free signed certificates.