I cant go there from here because I dont want to mess up my gmail account, but I'd start with making contact with google and go from there. I would check any and all systems and devices that the gmail was accessed from for keyloggers and malware/virus's as well as the most common way that accounts are stolen are by weak security reset info.
Such as whats your favorite color and what is your mothers maiden name is probably 2 of the worst security reset questions out there. The color one being the absolute worse due to dictionary attack methods and a short list of the most basic of colors.
On security reset questions I find it best to use answers that have absolutely nothing to do with the question. Such as ...
Q. What is your favorite musical instrument?
A. FriedClams
No hacker in the world would ever come up with that because they are too busy hitting it with a dictionary attack of different instruments.
https://accounts.google.com/signin/recovery