Employer does not like see the gaps in your resume. So first point that you need to remember that do not address this issue in your cover letter. It is not good thing to point out a defect or something negative in your resume. By mentioning it in your resume cover letter, you are dampening chances getting job.
You can deal with this issue in different ways. It is better t come with job gap issue during the interview. You can also use functional style resume writing where your resume is organized around the skills and projects that you have done. Though effective, this style of resume writing is not popular these days.
Do not emphasize on date while writing your work experience or you can use smaller font. In career accomplishment section, only mention the skills and accomplishments rather than date.
It is not a sin to have gap in your professional career. Do not try to hide anything from the prospective employer. As most of the companies will take up references, there is chance that eventually you will be caught. Try to convince your potential employer about your ability to handle difficult situation instead of discussing the past.