Regulations Regarding Eligibility:
1. Gratuity is given if the individual has served continuously for 5 years; in the 4th year, it should be more than 240 days.
2. In the case of death or permanent disablement, the 5-year count is invalid.
3. Now, for the calculation of gratuity, you need to know his/her basic + DA. CTC figures wouldn't help in calculating gratuity in your case.
4. It's calculated on the basis of 15 days per completed year of service.
5. It's divided by 26, as in gratuity calculation, we consider 26 working days for a month, not 30.
6. The formula is: (basic + DA) * 15 * number of service years / 26.
For more elaboration on gratuity norms, refer to any legal book.