How to unlock GONG account on iMac?

I don't know what a GONG account is, but if you've bought a second hand Mac and it does not guide you through setting it up when you first turn it on, then it likely was not properly prepared to be transferred to a new owner (you).

If you find that it is locked and asking for passwords when you try to set it up, it is going to be near impossible to reset without the help and information of the previous owner.

Return the Mac to wherever it was you bought it and ask for a refund.

Never buy a used Mac from an unknown source unless you're a gambler and willing to risk spending good money on a brick!

Yes, it is possible to take it to an Apple store for help. You can get clarification about the problem. If your trouble is not related to the previous owner then the people there may be able to help you get things straightened out. In-person help is usually much better than trying to get help from someone who can't put hands on the computer.

If you are in the US you can make a Genius Bar appointment for hardware/software service and support using this link – Contact Apple for support and service - Apple Support

If you can describe more clearly just what problem you are having, we might still be able to help here.

Exactly what Mac, model and year is this? And what version of macOS is running?

If the Mac starts up and goes to the desktop, you can find that info by clicking the Apple menu () at the top-left of your screen and then select About This Mac. (Note- for privacy and security please never post your serial number.)

AppleCare will do you no good at this point, consider this a lesson learned on why you should NEVER buy a used computer from an untrusted source. IF you cannot get your money back, it's gone for good and you have a computer you cannot use. Only buy used from reputable resellers such as Macsales.com or even better buy an Apple Refurbished Mac directly from Apple.

You're welcome, my friend. 🙂