I have the same problem on Nifi 1.5 and would be very interested to get the solution to get Nifi's En(De)crypt processor to work with PGP.
In the meantime I turned to another solution by using the ExecuteStreamCommand processor and outsource the decryption to the CLI which is verified to work:
Just be aware that you have to import the pub and private keys into the /home/nifi/.gnupg folder of the nifi user since that is the one executing the stream command.
So you might have to run these commands (on every Nifi node!) first:
gpg --import < pub_keys_armor.pgp
gpg --import < priv_key_armor.pgp