Does anyone know if a dark roux made from gluten free flour would have same taste and thickening properties as one made with regular flour?

A dark roux often does not thicken. It provides flavor though. So your gluten free flour might work.

I've done it with various gluten-free flour blends to make basic white sauces like gravy or bechamel -- so I bet it would work for a dark roux. The brown rice flour that is in most blends should get darker/more flavorful, and as PieceOfLayerCake says, you'll know if it worked before you add all the other ingredients. You may have to adjust the amount of liquid you add (depending on what kinds of flours/thickeners are in the blend you're using), but I say go for it!

I've never done that before either, but I somehow don't think that will have either the same browning or thickening properties. When I bake with gluten free blends, they certainly don't brown as well without added sugars....You should try a small batch with the fat, gluten free flour blend and water. I'd be curious to see what happens.