As an employer, you are not bound to pay gratuity to employees who served less than the qualifying period of 5 years. The employees cannot claim the amount either.
If you have provided a sum every year with a view to finding funds available for the payment of gratuity, by any means including investment in mutual funds as are available nowadays, the same could be regarded as an appropriation of profit (with a debit to Profit & Loss Account). If the same is not utilized, naturally, it will remain as your fund with all attributes of undistributed profit. It will not lapse but will be an investment only.
Regards, Madhu.T.K