Encryt a single user, not the whole device My android is multi-user. It is crazy to encrypt the entire device. How do I encrypt just a single user's account?

More specifically, the only thing I am trying to protect is my gmail account. 

I know it should be possible, as I have such a feature on my Debian Gnu/Linux laptop. It would however require integration with the login manager.
Afraid not. We're talking about "device encryption" here, which – as the name suggests – encrypts the "device" (in this context, the "storage device"). All users have their data on the same storage device – so either you encrypt it or not.

While it's not possible to "encrypt a single user", there are possibilities to encrypt selected files and directories, which might be an acceptable alternative for you.