Back up all your data, then open your google account from desktop pc or laptop. Change all the necessary settings such as password, recovery email, phone number, enable 2FA. Now, do a factory reset on your mobile. I would advice you, once you are done with this, create a whole new Google account with strong settings and use that in future. Also, check your applications whether you have any unknown application installed?
Also, if your Google account have been compromised then change the passwords of different accounts as well, only if you were using the "same" password on every account. The reason being if they can access your gmail with the password they can identify your other accounts in a matter of time.