'Gravity' spots on leaves

Whenever I use 'Gravity' my plants get what I call: 'Gravity' spots. I have become used to them, and almost welcome the look because I know that it means that harvest is near. I use it full strength, and I raise my lights. I followed the directions religiously.
but...
What causes it? There must be a nute in it that burns when the light hits it, or something.
Any info is mucho appreciado.

I guess no one uses Gravity. Hmm.