Hello,

Gap certificate is an affidavit on a non-judicial stamp paper stating your reason why you took the gap year. Gap certificate is not required for NEET examination. Although in some colleges in certain states require you to produce gap certificate while states do not. It is not compulsory but then again you never know so it is  a  1-2 day process so it is better to be prepared specially because you took a 3 year gap.

Hope it helps.

Good luck.