Have you informed the company about the outstanding dues?
Non payment of salary and gratuity can be challenged by going to Labour Commissioner.
Company has accepted the resignation and now refusal to pay your legally entitled dues is just a ploy to save some money at cost of employee.
Write a clear polite letter placing all dues from company on record and ask for it to be paid immediately.Send the communication by Speed Post so as to have proof of delivery to company.
Mental torture is not easy to prove, but non payment of legally entitled dues is easy to prove.
Collecting outstanding dues from customer may be part of your job responsibilities but cannot be condition precedent to payment of your unpaid salary and Gratuity upon your resignation which has been accepted without any conditions by the company.
As suggested, write polite but firm letter giving details of your dues and payment of the same within 7 days failing which you will approach Labour Authorities including Court for recovery of the same with interest.
You lodge a complaint that your wages for december not paid by your employer in local labour office. You make an apllication in person or send by registered post to your employer for payment of gratuity, wait for considerable period. If no payment is being made, you lodge another complaint for Gratuity in labour office.