Gradient effect makes light display different colors and brightness at once along its multiple segments.

Only products built with individually controllable segments, such as RGBIC lightstrip, Neon Flex Strip, Fairy lights, Gradient Floor light, Gradient Bar lights and Outdoor String lights, support gradient effect. 

If you apply a light mode with gradient effect enabled on a product without individually controllable segments, it will just play the selected dynamic effect (one color at a time).