@Kanika Mathur you can reach out to K A Pandit, they are one of the oldest acturial firms and can help you in your questions related to managing gratuity fund.
https://www.ka-pandit.com/index.php
As mentioned by all, every year there is a actuarial valuation done based on the various factors like the salaries, age, tenure of the employees and an amount is decided, you need to ensure you have that amount in the trust fund. There are a few banks who also manage trust funds, but preferable according to me is LIC. They are very good in managing it, we do it through them.