https://player.vimeo.com/video/655327283?h=2fcd0fc1f9&badge=0&autopause=0&player_id=0&app_id=58479
The effect of tfGradient operating on tfTerrain, with a material and a -Subtract Mode set in the Operator tab, driving the gradient.
There are five modes: Radial, Radial, ex-center, Linear, Square and Rectangular.
The mode is set at Radial, by default, which is a round gradient, as shown in the images above.