Hey Ankit! 

GAP certificate is nothing but your declaration given on rs. 100/- non judicial stamp paper stating the reason for gap in between education. In case you have a gap in your studies the same can be covered and allowed by drafting and computer printing an affidavit ti the effect by stating as to what all courses or studies or any other occupation that you followed in the gap period. The reason should be a plausible one. Get the affidavit attested from a notary or a gazetted officer and submit the same for covering the gap.

Yes, you must take it with yourself for the document verification process. It is important.

I hope this helps you!