A gradient is defined by dy/dx.

And it's associated with lines.

If you have gradient=2, you'll have a line that goes through origin and (1,2).

If you increase the gradient, it'll decrease the apparent 'dy'. You do it until the line becomes vertical, equivalent of dy=0. A vertical line is a valid object, but mathematically, it's no longer a function. You might say your math breaks describing the thing so you'll need a different math.