Can I substitute regular all purpose flour for the gf flour here?

I think your best bet is to search for Kim Boyce's original recipe from Good to the Grain, which is not gluten free. Here's the dry mix ratio from Kim's book:
1 c plus 2 tablespoons barley flour (this is where you sub buckwheat)
1 c all-purpose flour
1/4 c dark brown sugar
2 teaspoons baking powder
1/2 teaspoon baking soda
1 1/4 teaspoon Kosher Salt

All of the GF flours listed in the GF recipe are significantly more water-absorpent than conventional AP flour, so I am thinking that a direct substitution will not work especially well. Here is my basic scone recipe. Rhubarb is quite acidic, so if you add it to the mix, measure how much and also sift into the dry ingredients baking soda in the ratio of 1/2 teaspoon for every 8 ounces of rhubarb.
http://food52.com/recipes/11019-cream-scones-sweet-or-savory

I saw this question and hoped someone who is expert on GF flours would weigh in. I am not, but I looked at some articles online and the general consensus is that you can sub AP flour for the gluten free in equal proportions. The good thing about a recipe for scones is that you can feel the dough and know when it's right, you will know if it's too sticky or too dry. If you want to make scones using regular AP flour there are some fantastic recipes on this site, you can add the rhubarb just like in the recipe you want to make.