Hi,
Gratuity could be used more as a retention strategy. Though it is for old-age security and one of the government acts to provide financial security, nowadays it is losing its importance fast as people resign before completing 5 years. Therefore, you may consider including in the appointment letter, "Gratuity will be payable as per the act."
However, only if someone completes 4 years should you start reminding them that they will be eligible for gratuity if they complete at least 4.5 years - as one of the retention strategies for the company. In one of my previous companies, which was a start-up, we only bothered to open a gratuity fund with some minimum amount when the first employee completed 4 years.
Thanks,
Geeta