According to your statement,the final settlement includes gratuity payable and this was not paid.
First make claim of gratuity to the employer as per Gratuity Act duly calculating the amount payable as per the act.While doing so please give details like last pay drawn,dateof joining,date of leaving and caculation of gratuity etc,.
Please give them reasonable time to act on your letter.
If the claim is not settled then make a complaint to the Commissioner of Labour of your jurisdiction alog with a copy of the claim you made to the employer.
If possible please also make a claim through e-mail to Central Labour Commissioner with copies of all correspondence.
Simultaneuosly send a notice to the employer through an advocate claiming the amount and giving time to respond after which you can file a case with the Civil Court of your jurisdiction.
Filing a case is the last resort and should normally get selled before that date.
kkm