The Payment of Gratuity Act 1972 is a complete code in itself and empowers the Controlling Authority under the Act to be the authority to decide the question as to who is to receive the gratuity. The Act enjoins the establishment to pay gratuity to the nominee when the employee is no more. In this case if there is a nominee, you can deposit the gratuity amount with the Controlling Authority giving the details of the nominee and the family. You can also mention the rival claim received from the second wife.
It may not be advisable to wait for the succession certificate as the process takes time, and the employer is liable to pay interest @10 % for the delay in payment beyond a month, in case it is challenged.