I finally got it to work. A simple ball with a red ambient color and ambient value set to 1 (I left the rest at default values), resting on the ground plane (shadow catch only off, texture map connected). Default Gather node plugged into the Ambient input of the ground root shader, ambient color white and ambient value set to 1. Works. The Gather node also works when plugged in Translucence, Alternate Diffuse or Alternate Specular. It won't work in regular Diffuse or Specular.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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