[important]

Assuming hacker did sign in into your Gmail , you might be able to get that information from the list of last logins in your Gmail account. Any IP that's out of your normal location would reveal that. More in this link https://support.google.com/mail/answer/45938?hl=en

[unicornporn]

Yeah, this is an account that only forwards emails, so I almost never log in. However, when I changed my password now I logged in and out a bunch of times. This made this very short list of recently logged locations only contain one line that was not from today. Hmm. Would be better if they showed 50 recent logins or something...