I've been using the Mailvelope plugin for Chrome (there's also one for
Firefox) to manage PGP keys. It integrates with Gmail (and other webmail
clients) and, in my opinion, has quite good usability.

https://www.mailvelope.com/

The problem then becomes finding others who also have PGP keys and
encouraging your friends and colleagues to encrypt. You need a certain
critical mass of people who use the technology for it to be anything but a
curiosity.

I also have a Protonmail mail account, which does end-to-end PGP encryption
with other Protonmail users. Emails sent within the system are
automatically encrypted, since Protonmail itself manages and applies the
public PGP keys. Private keys are supposed to be encrypted within your
individual account using a passcode you have to enter when you first login
and therefore not accessible by Protonmail employees.

https://protonmail.com/

Again, the problem is that I don't correspond with anybody else who uses
the service.

You can also share your Protonmail PGP key so people not using that mail
system can send you encrypted messages. The big omission is that they have
not provided a built-in keychain in Protonmail so you can send encrypted
messages back.

Back in June of 2014, Google announced its End-to-End encryption plugin for
Chrome, which was intended to add Protonmail-like encryption to Gmail, but
two years later it is still not released.

https://github.com/google/end-to-end

Tom Keays
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*Key*: http://tomkeays.com/files/pgp/gmail_public_tomkeays.txt
*Fingerprint*: 7DEB D2DE B079 EE61 5401 784D D741 00E2 BAE9 03A7