Assuming these hypotheses:
1) Your account isn't hacked (hacker has not entered your account directly)
2) Gmail encrypts via SSL end-to-end
3) The hacker is not Gmail or Google
4) There is no access to the machine from the hacker - he cannot install anything in your machine (no certificates, no keyloggers, etc.)
It is just your personal house: computer -> router -> ISP -> Google.
Is it possible for a person with some kind of program (via man in the middle or other method), to decrypt and read messages?
Maybe the password wasn't strong enough and in fact, it was hacked via brute force or social engineering?
I have confirmation that this action happened to a person, but I really do not understand how this can be done (maybe faking the SSL certificate?).