Knowing Kellers stuff, its propably as optimal as gluten free flour can be culinary wise. He is obsessed perfectionist, makes many paleos pale by comparison ;)

I suppose if you are only concerned with gluten, then, well, it's gluten-free.

Cornstarch - not paleo, might be tolerated
white rice flour - not paleo, but acceptable under PHD (safe starch doctrine)
brown rice flour - even PHD would say to avoid
milk powder - not paleo, might be considered Primal
tapioca flour - safe starch?
potato starch - safe starch?
Xanthum gum - best to avoid weird ingredients (precautionary principle), but YMMV...

Banded Girl is correct. It's not paleo, definitely sub-optimal and possibly bad for you

What did you want to do with it? (We could probably come up with some decent alternatives)

I agree that this is a suboptimal flour choice. If you really want baked goods, try learning to cook with coconut flour. Almond flour, almond butter, sunflower seed butter and pumpkin seed butter all can be used for baking also. You can find lots of fabulous recipes on many of the Paleo blogs. Here's a Paleo quick bread recipe using Sunbutter as the base from my kid-friendly low oxalate blog (mostly Paleo/WAP).

http://lowoxalateinfo.com/sunflower-spice-paleo-quick-bread/

And a recipe for Paleo pancakes using coconut flour.

http://lowoxalateinfo.com/paleo-pancakes/

Keep searching for healthier recipes for the occasional baked good. I have used a few coconut flour recipes that also contain potato starch or a little rice flour. This is a very satisfying combination and will at least keep you in the "safe starch" realm.

Jeanne's Gluten Free All-Purpose Flour Mix is simpler and avoids the corn. http://www.artofglutenfreebaking.com/2009/11/the-story-behind-my-gluten-free-flour-mix/

• 1 1/4 C (170 g) brown rice flour
• 1 1/4 C (205 g) white rice flour
• 1 C (120 g) tapioca flour
• 1 C (165 g) sweet rice flour (also known as glutinous rice flour or under the brand name, Mochiko)
• 2 scant tsp. xanthan gum

Xanthum gum is a natural sugar (polysaccharide) created by a bacteria by fermenting corn. It's a natural thickener.

If you eliminated the brown rice flour, you would have a "safe starch" flour. But not Paleo, because it is still grain-based and processed.

Making things into flours increases the rate you absorb them so there are insulin/glucose consequences beyond where the starch is derived from. Also, things made with flour tend to be carbohydrate intense and not very nutritious.

I'm sorry, but I have to ask. If your commitment to "paleo" is supposed to mimic a supposed ancient diet, how would using Cup4Cup, or any other "substitute" that you make from processed starches, advance that goal? I'm not against C4C or any other such product, but then again, I eat cultivated plants and processed foods.

If Keller has endorsed it, I would try for for making a rare dessert to be enjoyed by those who have not bought into Paleo but is gluten free. Or for suggesting to a family member who is trying to cook me something gluten free.

I've used an Arrowhead mix GF a few times. C4C looks similar but Arrowhead is 70% the cost. Worth the premium to me if it tastes better.

Arrowhead Mills All Purpose Gluten Free Baking Mix: Organic rice flour, whole grain sorghum flour, organic tapioca starch flour, organic whole grain sorghum flour, leavening (sodium acid pyrophosphate, baking powder, monocalcium phosphate), inulin, rice bran extract, xanthan gum