Dear Sumit Barua,
Group Gratuity Policy is, in fact, a contract of insurance only in between the Insurer viz LIC and the employer though the beneficiary is the employee covered by the policy. As such the employee can demand the maximum sum of gratuity only as per the Payment of Gratuity Act,1972 and not the maturity value of the insurance plan. When the gratuity is not paid as per the provisions of the PG Act or delayed, of course he can proceed against both the employer and the Insurer. The insurer's liability is limited to the premium paid by the employer.
If your salary structure comprises of different components such as basic, dearness allowance and other allowances, only the sum total of the last drawn basic and D.A alone would be taken for computation of your gratuity as per the Act on the termination of employment and not on the basis of the premium paid; if the component of D.A is absent in the wage structure, only basic is to be taken into a/c and in case it is consolidated, the entire sum has to be taken into account.
In case the amount of gratuity paid by the employer through the LIC is less than the amount as per the Act, you can file a claim u/s 7 of the Act to the Controlling Authority under the Act.