In your scene the object
Cube.001 is still being rendered. Since the cube has a white material the light casts shadows which cause the grey gradient.
There is a difference between temporarily hiding/unhiding in viewport (
Alt+H on keyboard), permanently hiding ("disabling"), and hiding ("disabling") in renders. I assume you desired the last option and accidentally used the first.
In the Outline editor you can click on the funnel icon on the top right to enable more filter controls per object/collection. The third icon is the "Disable in Renders" function. When you enable this option for
Cube.001 the rendered image has a black background.