How can I fix this gap in my kitchen counter?
The screws attaching the cabinet to the wall are placed at every stud, near the top of the cabinet where you see the solid wood. Studs are typically spaced every 16" and can be found with a stud finder. The cabinets will be attached to each other, so look for screws in adjacent cabinets, too, tightening everything uniformly. There should also be something holding the cabinet down to the floor, behind the toe kick on the front.

What you need to find out is why the cabinet is pulling away. Perhaps it's just caulk drying and shrinking. It's possible your floor is settling/sagging in the middle of the room. Or the outside walls could be going out of plumb. Some of these, like sagging floor joists, could indicate other problems that need to be solved first.

To fix the problem, you could remount the counter top tighter to the wall. But 99% of us would simply add more caulk.