Everyone has gaps these days because of redundancies, trouble finding work, taking time off to have children, and many employers won’t really care if it is because of these reasons.Sometimes, illness can be cause for a gap, for example cancer or depression. If this is the case, mention in your CV ‘Career Gap between xx/xx and xx/xx due to ill health. Disability laws state that they cannot hold illness against you and you do not have to discuss it unless provisions are required for you to be able to do your job properly (for example, wheelchair ramps). If the time away from work is for negative reasons such as imprisonment, do not place any emphasis on the gap, and if they ask about it, be honest but explain that you have rehabilitated and have references to confirm that. A decent employer will hopefully see through it but one can only try.
Whatever you do, DO NOT LIE about it. Lying on CV/cover letter/during interview can be a sackable offense if the truth ever comes to light!