postinst is in pgpgpg 0.13-9.1.
This file is a maintainer script. It is executed when installing (*inst) or removing (*rm) the package.
The actual contents of the file can be viewed below.
1 2 3 | #!/bin/sh -e
update-alternatives --install /usr/bin/pgp pgp /usr/bin/pgpgpg 20
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