You don't need to trust any other authority ..
you can get your own whitelist ..
- adding someone to your whitelist is equivalent to signing someone else's public key in pgp/gpg
- trusting someone else's whitelist is the same as trusting someone that has a signed key from a friend that you have signed as well.
If you want to link people with a gpg key, then go for it. but using PGP ontop of steem only adds redundancy that could also be added to the protocol with whitelists.