If you had been relieved properly, and had received all the employment documents, then you shall raise a complaint/application before the Controlling Authority as appointed by the Government.
Please find the details according to the The Payment of Gratuity Act, 1972, given below.
RECOVERY OF GRATUITY. - If the amount of gratuity payable under this Act is not paid by the employer, within the prescribed time, to the person entitled thereto, the controlling authority shall, on an application made to it in this behalf by the aggrieved person, issue a certificate for that amount to the Collector, who shall recover the same, together with compound interest thereon at such rate as the Central Government may, by notification, specify, from the date of expiry of the prescribed time, as arrears of land revenue and pay the same to the person entitled thereto :
Provided that the controlling authority shall, before issuing a certificate under this section, give the employer a reasonable opportunity of showing cause against the issue of such certificate. Provided further that the amount of interest payable under this section shall, in no case exceed the amount of gratuity payable under this Act.
Do mention in the email something to the effect of "A copy of this correspondence is being sent to your good office by Registered Post/Speed Post for your due consideration." and do so. Maintain records of this Speed Post (booking receipt, and delivery report as available online) AND email in hard copy.