Tierion creates a Merkle tree of incoming hashes and puts the root of the Merkle tree on the Bitcoin blockchain which proves that the hash was placed there prior to the time embedded in the BTC transaction. You want to use their HashAPI.
https://tierion.com/features Other similar services are: http://truetimestamp.org https://proofofexistence.com These services don't need GnuPG, but nothing to stop you from hashing a signed document.
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