A GradientLight is an environment light. The environment color is generated using a gradient.

The left of the gradient is the top color, the middle of the gradient is the horizon color, the right of the gradient is the bottom color.

Edit the gradient of a GradientLight
  1. Locate the GradientLight in the NodeList view.
  2. Open its EnvironmentMap and Shader children.
  3. Select the Gradient child node.
  4. Edit the gradient.
The User Guide/Lighting/Environment lights section.