On a resume, it is best to list the jobs that are relevant to the one you're applying to gain, regardless of timeline, and leave off the jobs that have nothing to do with it.
For vague example, if it is an office job you might list similar jobs from 19, 11, and 4 years ago, but leave off the waitressing job 7 years ago and the part-time retail clerk job from 12 years ago.
If the job you've held for the past 15 years is the only one relevant to the new position, it is the only one you need to list. If you had a similar job 20 years ago, then list that too.
Timeline gaps are not a big questionable deal in most cases anymore, and cover letters should usually be very brief to non-existent unless requested in the job offering. They are often not read at all, and writing a full page may get your resume binned.