Hello

Not every college asks for affidavit. If it is mentioned as one of the documents needed only then you have to submit the affidavit. You  can get an affidavit from the court (sub diviaional or divisional) or through google. In some colleges a written and self signed note regarding a gap year is enough so it all boils down to which collge you are joining. According to me, you should mail them with the doubt. All the best!