Google is planning to introduce end-to-end encryption to Gmail by implementing more complex encryption tools such as the secure PGP (Pretty Good Privacy) PGP encryption could be problematic for Google as it won’t allow them to scan your emails to serve its spam filtering feature, content-based advertisements and even the search option. PGP protected emails would require decryption keys that only the sender and recipient would have to have to read the content transmitted between the two, and ideally Google won’t have access to the messages.
Source: https://thehackernews.com/2014/04/End-to-End-Encryption-for-Gmail.html