The rule is simple. Once your RP has been transferred to a new company, the previous company loses all its controls over you. Just refuse to sign any documents they are asking you to, and they can't do a-n-y-t-h-i-n-g to you now. If they threaten to stop your gratuity, just give them a counter threat telling them you will seek the help of the authorities in recovering your money. Follow this action by sending them an email asking for them to pay you your gratuity immediately. If they refuse, follow up with a another email telling them you will seek the help of authorities in recovering your dues. Keeping things in writing will support you should the matter reach the authorities.
It is evident from your post the HR people at your previous company are not professionals and do not know their job.
Take a bold, manly stand against them. You have nothing to worry about. You will be the winner in the end.