To decrypt in the browser, you must be logged in,
and you must host your private key in Keybase's
encrypted key store.
If you don't know what Keybase is, hi there, read this welcome message.
keybase pgp decrypt -m "message"
You may import from Keybase to GPG easily and then perform
whatever cryptographic actions you want.
# using curl
curl https://keybase.io/[them]/key.asc | gpg --import
# using `keybase pgp pull` which
# imports to GPG key chain for you
keybase follow [them]
keybase pgp pull [them]