HN user
I've built a similar piece of javascript crypto: PasswordProtectMyFile.com. It's a single HTML page that can work with internet turned off. The ciphertext is another HTML page with all the javascript needed to decrypt itself.
It's open source: https://github.com/louissobel/ppmf
It's all in one html file, so it will work disconnected from the internet. Download or save the page, turn off your internet, open it in a browser, and encrypt with peace of mind (although the javascript could still use off-line storage and then next time you're online on the page access it, I guess).
It's a little clunky, but I do have a process in place for checking that the website matches the code: https://github.com/louissobel/ppmf/blob/master/bin/check_pro...
Tested and working with recent Chrome, Firefox, Opera, Safari 6.1+, IE 10+. https://www.passwordprotectmyfile.com/faq.
It uses, among others, the Blob API so will not work on older browsers.
The encrypted result of this tool contains all the code necessary to decrypt itself (in a web browser).
I'm not too familiar with GnuPG, so I don't know if that's different.