I have two of King Arthur Flour books--the latest is KAF Baker's
Companion. I like it best. I have not had a problem using Gold Medal
Flour for KA flour. In fact, in this book, it does not specify the
company's products--e.g. many of the recipes call for unbleached
all-purpose flour, no specific brand is suggested. I use my Gold Medal
bread flour because that is what I have. I have not had any problems
with the quality of the baked goods.

I do have to make the same adjustments with their bread recipes that I
have to make with those from other books--namely, if the recipe says 4
to 5 cups of flour, I start with a scant 4 cups and add the rest of the
4 cups' worth if needed. This is because I live in the Southwest which
has a very dry climate. When I lived in the Pacific Northwest with its
high humidity, It always took the larger amount. This is/ was true with
any brand of flour.