Great question! NOSTR could potentially use GPG (Pretty Good Privacy) for public key cryptography, but there are some reasons why it doesn't. Firstly, GPG is a specific implementation of public-key cryptography, and NOSTR aims to be protocol-agnostic, allowing users to choose their preferred client and encryption methods. Secondly, while GPG is widely used, it has been criticized for being less than user-friendly, especially for new users. By using plain JSON objects