I've used it before for grounding and the AHJ passed it. I'm not sure what testing UL is going to do on a 100% copper solid bare conductor?
For that matter, since a ground can be "bare, covered, or insulated", other than smoke producing properties (a concern no matter the use), the only reason I see for the gray not being acceptable is the color-which needs to be green or green with a yellow stripe.
Thought you might find the description interesting-bare cooper for overhead transmission wiring. I've ran into some recently-I honestly didn't know the #4 bare was marketed for such use!