Unfortunately, if you don't know the Gmail username and password, the only way to unlock your phone is to perform a factory reset. But, along with this process all your settings will be deleted as well as your applications that you have installed on it. So, first you need to shut down your phone and wait 10 seconds. After that, press volume up + home button + power button all together. Next, search wipe data/factory reset and press OK. Reboot your phone and it will be unlocked.