What exactly is the problem? If it's just the unsightliness of a gap that collects dirt etc., I would clean it out and fill it up with either pebbles or a crushed material. If the color is like a very dark brick, so much the better.

If the problem is anything other than visual - e.g. perhaps drainage or structural - then please spell it out.

Drainage + structural. The right adjoining wall is a brick enclosure for the stairs to the basement. A gap like this, will over time undermine a foundation, trickling into everything around it. Currently, the water leaks down from that gap which is about 9' long (all broken or easily movable by hand) then reappears two feet over leaking down the stairwell that leads to the basement. I have water damage all around the brick work from water getting under it and burrowing a path, breaking the mortar above it.

The substrate under the patio has failed. Remove, fix the substrate and drainage and replace.

It seems this is a PRO forum, all that is beyond my experience. Thanks for the responses! FYI: I was thinking of a rubber sealant that would fill the gap, forcing water to flow along the natural grade.

OK, backer rod and the rubber sealant of your choice.

The gap is much too wide for sealant and sealant won't last in a horizontal position especially if water collects there. What you need is metal flashing attached to the wall turning out on to the patio possibly with a curb on the patio.

You might consider a drain pipe under the patio.