Gmail uses Transport Layer Security (TLS) to encrypt messages in transit between Gmail servers and other email servers. This means that messages sent from one Gmail account to another Gmail account are encrypted while being transmitted over the internet. However, once the message reaches the recipient's inbox, it is no longer encrypted, so it can be read by anyone who has access to the recipient's account.
In summary, Gmail-to-Gmail messages are encrypted in transit but not at rest. Google also offer additional encryption options such as S/MIME or PGP.