What is the most practical way to achieve this?
Unfortunately, there is currently no reasonable way to use GPG/PGP from within Gmail directly. The only two options are to encrypt the message manually from a client-side tool, pasting the result into Gmail when you're done, or to use a desktop mail client with GPG integration (such as Thunderbird + Enigmail) to send messages you need to be encrypted. Decrypting would likewise entail downloading the message from Google as raw source, running it through the desktop client to decrypt, and reading it there.
There is now an alpha-quality Chrome extension for using GPG in Gmail. I have not tried it, so I cannot vouch for it.
FYI "apps and extentions can not be added from this website" http://thinkst.com/tools/cr-gpg/
Came across a very promising looking Chrome extension for sending and receiving GPG encrypted emails: mailvelope.
It supports a variety of webmail providers.
The best Google Chrome extension for this is: "Mymail-Crypt For Gmail"
The Decrypt and Encrypt buttons are well located and easy to use:
such as the managing interface, plus doesn't require you to have gpg installed in your computer.
Google has a plugin for Chrome called End to End which will allow exactly that. However, it's currently in alpha, and has to be compiled yourself. There's no word on if it will ever be an official part of chrome.
I came across GPG4browsers. It's possible to check a signed messages and I can import my private keys. Apart from that it's still in an early stage.
FlowCrypt is another browser extension. I'm not sure about any advantages or disadvantages it may have compared with Mailvelope. I read somewhere it supports signing messages but I haven't tried whether mailvelope also can do that.