You cannot make perfectly vertical slope in landscape.
You have 2 options:
make size of cell smaller, but this gets really heavy fast.
make some static meshes that can cover gap, then make material that tiles seamlessly with landscape (use same uvs in both (like world coordinates with same scales)
However what everybody is really doing, is covering this gap with random rock/ground meshes.
You can always make landscape lower there, and hide all that elevated surface with meshes.
Do not worry about such gaps at very beginning, you will cover everything with meshes later.