実際のところ
というわけで……(?)
passphrase-fileをつかって、
$ ssh robot@192.168.0.13 bash <<SHELL > gpg --batch --passphrase-file password.txt --output example99.txt --decrypt testdata.txt.gpg > SHELL robot@192.168.0.13's password: gpg: encrypted with 3072-bit RSA key, ID FB7BFEE9533DFC94, created 2019-08-17 "encrobot <encrobot@example.com>"