your gpg example removes the deniability since it proves who wrote the message.You confuse digitally signing a message with the sender's private key, and encrypting a message with the recipient's public key.
A shared secret implies that the message was sent by someone who knows the shared secret, and that restrict the number of potential senders.If you mail a message with gpg, everyone knows the public key, and the message is still safe.
The knowledge of a shared secret suggests that the message could be authentic. It makes more sense to use asymmetric encryptions (without signatures).
Proton's CEO comments on Trump: https://medium.com/@ovenplayer/does-proton-really-support-trump-a-deeper-analysis-and-surprising-findings-aed4fee4305eI use Proton and I'm happy with it. Tuta seems a great option but it's less complete if you need the full package.I recommend to buy your own domain.