Hi and Welcome. My Name is ***** ***** I look forward to assisting you today with your question.
Try resetting the Gmail password ***** here --> https://www.google.com/accounts/recovery/
If the phone is not accepting the password, then your only option is factory reset. Factory reset will delete everything from the phone and start it fresh. Unfortunately this is your only option if you are unable to unlock it with Google account. Let me know if Gmail doesn't work
If resetting the password *****'t work, please let me know by replying in here and I will send you instructions to reset it.
To make sure, is this an old Galaxy S2 model ?