Your cars vs. traffic test is a good one. I agree with all of your answers except for "less" and "less and less." Both of these quantifiers are uncountable.
This is actually a very common mistake that even native English speakers frequently make. "Less" and "fewer" are both words which mean "a smaller quantity," but "less" always refers to something ...
If I were a consumer, and I read either of the two examples, I would suspect the product contained gluten. Why? Because the the label, as Shakespeare might have written, “doth protest too much.”
“This product is gluten-free” is perfect. I would believe that.
Do you like [food] is a general question not, a specific one, so it is very unusual for the [food] to be singular. It is generally plural (Do you like oranges?) or uncountable (Do you like fruit). In neither case is an article normal.