Hi,
Firstly, it depends on the no. of employees your company has, it is more than 10 employees that means your company is liable to pay gratuity to its employees. Gratuity is payable on last drawn basic / basic +DA salary (if DA is a component in your salary structure).
It dosent matter if it is mentioned in your Appointment letter or not, neither does it make a difference if it is not mentioned in the HR manual.
If your company has more than 10 employees, it is applicable by law and the company is liable to pay.
Regards,
Shweta Gehlot