Did the poster pay a salary to the retired person who is also a pensioner for the service rendered by him to the educational institution? If the answer is 'Yes', then the same analogy applies to the payment of gratuity as well.
Drawing a pension for services rendered under the Government cannot be a disqualification for reemployment in any private institution. Gratuity is a statutory payment payable to the employee by the employer on account of termination of his employment after completing the prescribed minimum qualifying service. How can a subsequent employer derive the benefit of not paying gratuity just because the employee derived the same benefit from someone else? Even if the employer had paid gratuity to the employee upon attaining the age of superannuation, if he re-employs the same employee, it is a different spell of service entitling him to gratuity when he completes the minimum qualifying service during the subsequent spell and resigns or is sent out by the employer for whatever reason other than dismissal for any moral turpitude or riotous behavior.