Dear Gopal,
You cannot simply write a letter to the Labor Commisioner or some other officer under his control for non-payment of statutory gratuity payable to you by the employer on the termination of your employment.
There is a procedure prescribed under the Payment of Gratuity Act 1972. The employee has to file a claim before the Controlling Authority appointed under the Act for the area where you worked last. It is a quasi judicial proceedings. Better, engage the services of an advocate for assistance including filing a claim and conducting the case on your behalf.