I am selling my old iphone4s but it needs me to factory reset it! Which I have done ! Now it requires my old Gmail account of which I cannot remember the password to

In order to overcome the forgotten passcode you must do a factory reset, it will wipe the phone clean and you will need to input all the data again.

Factory reset the phone!

This is an activation lock through Google. Basically, the device was factory reset, and there was a PIN or SWIPE lock screen and a Google account on the device when it was factory reset.

As a layer of protection and to ensure the phone is not stolen, it is looking for you to enter the Google account that the device was ORIGINALLY set up with. If you know the Gmail login, but need to reset the password, you will not be able to login to the phone for 72 hours AFTER the password reset.

If you do not know the login, there is no way to bypass this screen. You would need to get the Gmail account information from the original owner.

If you have more questions concerning this, here is an in-depth explanation of the security lock. Guide What Is Android 5 1 Anti Theft Device Protection Feature And How Do...

I’m happy to assist further over the phone at https://www.6ya.com/expert/christopher_d13aca62d29ed9ae

Connect phone to iTunes on PC and factory reset!

Depending on how your phone was setup. Your contacts should be saved into your Gmail contacts.

Since you lost your password, you will need to use the Google Accounts recovery webpage to reset your password. NOTE: You will need to answer questions to get back into your account. If your answers don't match up you can't get in.

https://www.google.com/accounts/recovery/

Try this before you do a factory reset by loading the firmware again.

If you set a pass code (or pattern) that you cannot remember, please enter the wrong password (10-20 times) until you are prompted to sign in via your google account. Please note, this will only work if you have already setup your google account on the tablet. If you have not set up a Google (or Gmail) account on the tablet, you will have to perform a firmware install, which will revert the tablet back to factory settings.

Support Troubleshooting Guide

Its sounds like when you did a factory reset it cleared your login information, just re-enter the email address you originally used for yiu Gmail account and the password - if you can't remember the password click the forgot password enter your email address in the box provided and you'll be emailed with a password reset. Once you've done that re login with the email address and the new password.

You'll need to go into the settings and add her account, then make that account the primary account on the phone, then delete your account. Here is a link (http://www.samsung.com/us/support/owners/product/SGH-I997ZKAATT) to a source of the owner's manual for your phone.

You want to go to settings/ accounts and sync/ manage accounts to set up her account. You may have already done so. However, best perhaps to wipe the phone clean before setting up her account. This can be done from the settings/ privacy/ factory data reset area, which will keep the google servers from using the account backups to restore your old account on the phone. However you will lose all installed applications by doing so. (See pages 120-121 of the gingerbread version of the manual.)
If you choose not to do a factory reset, follow the instructions for configuring your phone (http://downloadcenter.samsung.com/content/UM/201202/20120207034812540/ATT_i997_English_User_Manual_KL2_F3_Gingerbread.pdf
to add her account, then restart the phone, delete your account, restart again and sync her account and see if the phone won't maintain her settings.
I hope this information allows you to resolve this issue. If you need further assistance, please post back with a comment to this thread.
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You do not need gmail.com only tap on SETTINGS to factory-reset your tablet.