Dear Rajesh,
Gratuity is a one time terminal benefit which requires no periodical monetary contribution from the employee. Gross wages or salary is the sum total of the periodical monetary consideration payable by the employer to the employee for the services rendered by him to the organization. Employer's contribution to Provident Fund, ESIC,Gratuity Fund, Medical Insurance etc., are called Fringe Benefits. These fringe benefits are extra compensation in addition to direct wages or salary periodically payable to the employee. However, in order to ascertain the overall cost incurred in respect of individual employee, all sums payable are clubbed together which is called as C.T.C. Whereas statutory gratuity under the Payment of Gratuity Act,1972 is payable only on the termination of employment to the employee computed on the basis of his length of qualifying service and last-drawn wages or salary.
As such your statement that Rs.500/= being shown in the monthly salary slip as gratuity is something confusing. Whether it is shown on the deduction side or gross side of the salary slip? If it is on the gross side it forms part of the essential component of your direct wages/salary which is a practice unheard of so far; if it is on the deduction side, it is an illegal deduction. Seek your feed back.