Yes it is necessary to provide a gap certificate.

You can avail certificate from your nearby local police station or by post office.

This document is required to produce during the registration process when you reach for admission at your alotted IIT.

You can go through your concerned IIT official website and search regarding the format which they want or which they accept.

At MNNIT,they had accepted by just an application on a simple paper signed by a sub-inspecter or higher authority.