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 https://keybase.io/[them]/key.asc | gpg --import

# using keybase track, which imports to GPG and
# also audits and displays identity proofs
keybase track [them]