Using the Keybase command line app

keybase pgp encrypt -m "message" [them]

With GPG or another PGP program

You may import from Keybase to GPG easily and then perform whatever cryptographic actions you want.

# using curl
curl[them]/key.asc | gpg --import

# using `keybase pgp pull` which
# imports to GPG key chain for you
keybase follow [them]
keybase pgp pull [them]