I have used the GF Bisquik mix - I made a batch of pancakes and a batch of biscuits. I followed the recipes as written and they came out with similar results to the regular version (the biscuits were a little more crumbly). The reason I suspect for there not being shortening in the GF mix is the flours work so differently that a standard amount would not give you the right textures. Another possibility is many people are not just GF but have other food allergies/sensitivities such as corn, casein(cow dairy), egg, and soy. So depending on what else you need to use to make a recipe will change how much and what type of fat source you would use. Many of the standard Bisquik baking mixes do not use eggs in their recipes - however most of the GF ones do - for its binding properties and the yolk provides some of the fat in addition to the vegetable shortening/butter/oil to get the right mouth feel.
In order to get the desired mouth feel and texture in baking of any kind, Chemistry is the name of the game. Doubly so with GF/CF baking...eggs, tapioca flour and xantham gum have become my friends in places I would have never thought of before I started experimenting.
Check out the Bisquik website for some interesting recipes that have had the bugs worked out of them.
I'd buy it again- especially in a pinch to make the old standards.
"Hope is nature's veil for hiding truth's nakedness."
- Alfred Bernhard Nobel