That linear gradient you are using uses the uv’s of the object.
So depending on how the object uv’s are placed/rotated you will have the gradient behave differently. In the case of the cube for example if every face is stretched to the uv limits and rotated
the same way the sides will show the gradient as you described.
A landscape is simply a heavily subdivided square with x-y coordinates.
So as Host has explained you need to use the World position masked from B which is the Z component, Normalize to get a 0-1 black to white gradient to plug into a lerp 