Hey Christopher, I don't believe the postgres source supports pgp decryption, you'd have to modify the source code of the postgres connector to make this work with the pgcrypto module. My guess is that you'll have to implement a decrypt function in PostgresSourceOperations.java and then modify the PostgresSource.java file. But it's not an easy change.
I'd suggest making an issue on Github and perhaps someone from our community can chime in on a possible strategy for implementation.