Denial of gratuity

i have employed undet the management category directly reporting to the Chairman of a college in July 1998 on a monthly fixed salary paid by cheque. My bame appears in the college calender as a non teaching staff and I have also been issued an ID Card. My name was not included in the attendance register since I was given special exemption from signing the same with freedom as to my reporting time.

Now after 13 years of service, the Chairman takes a view that since I was not signing the attendance register( which was at the behest of the management) I am not a regular employee and hence I am not eligible for gratuity.

It may be noted that I have been paid a salary every month by cheque for the past 13 years and this is clearly on record in the finance department because I have also issued a receipt for salary every month.

What is the remedy open to me to enforce payment of gratuity ?

Please advise.