You can factory reset the device but you might have to login to the Google Account that was on it previously in order to complete the setup and make the device usable once again.
syaoran is absolutely correct! You will need to figure out which e-mail address you used for the particular device. I was trying to think what would be the best way to figure this out and if it were me, I would try using Android Device Managerhttps://www.google.com/android/find. If you do indeed have more than one Gmail account, I would log in to the site using each one and see which devices populate. I can't guarantee that it'll work, but it'll be your best bet for figuring out which e-mail address is associated with the phone.
Oh, and please note that if you can't remember the password to a particular account and you reset it, the phone may lock you out for 24-72 hours (your mileage may vary based on your version of Android) after the change.