https://www.quora.com/Gmail-is-not-allowing-me-to-use-the-old-password-What-should-I-do
There is a solution. Google save only last 100 password entry in their database and if you want to set password that falls into last 100 saved password then Google will not allow you to set it up.
So if you can spend 10 min time, changing password for 100 times with some dummy one, you can able to set the same password you wish to have.
I will give you an example to make it clear:
If you want to set "quora123" as a gmail password but you used it earlier, google will not allow you to do so. For that you need to replace last 100 password entry with some dummy one-
