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Sorry for the delay and thank you for your patience. My name is Michael. I will be helping you with your question today. Please let me know if there as any additional information that you feel might be pertinent to your question. If I don't hear back from you, I'll provide the best answer I can based on the information you've given so far. You will still be able to reply with additional information or questions at any point during our conversation, and I'll revise the my answer accordingly.
If you do a full reset and wipe. you do not need your previous password. When you go through the setup, it will ask you for your google username and password. This is the username on your gmail account. IF you don't remember it, you can go to gmail.com and reset your password. If you are unable to do that, then you can still setup a new gmail.com account and use that. In any case, it won't prevent you from using your phone. But if you do setup and use a new account, it will not sync data from the previous phone - that's by design for security. So you have two options.
Recover your original gmail account and use that
Create a new gmail account and use that
Either will allow you to use the phone.
Please let me know if you have any questions.