PGP = pretty good privacy. An open standard for email privacy.
GPG is Gnu Privacy Guard is the implementation - use it as client
Enigmail is an add-on for Thuderbird if for PGP
Both are end-to-end encryption. It will pass thru gmail but it won't look like gibberish if you have private key
Usual case - confidentiality - only 2 communicators hear each other
Integrity - noone can mess with it in transit
Authentication - only I sent the message. It's impossible for someone else to send an email in my name
Mozilla Foundation produces Firefox (web) and Thunderbird (email)
Thunderbird uses IMAP - when you mark it as read in Thunderbird, will be read in gmail
Enigmail is a good interface fo GPG
The Intercept - Glenn Greenwald - one of Ebay billionaires funded Poitras, Greenwald, Scahill

Pgp

See Also