Has anyone checked https://github.com/google/end-to-end ? I want to believe google did their due diligence.
Go's crypto/openpgp package (http://golang.org/x/crypto/openpgp) is probably one of the closest "full" implementations, although it's a library and not a CLI binary.
Are there any viable FOSS implementations of the OpenPGP standard other than GPG? Detached GPG signatures seem to be the most common mechanism to validate software distribution and similar.