If you do not have buttermilk on hand you can use ATK's buttermilk substitute!
Mix 1/2 cup milk with 1 1/4 cups yogurt (both whole-milk and non-fat yogurt will work) and substitute this mixture for the 1 3/4 cups buttermilk.
Substituting the buttermilk with a mixture of lemon juice and milk is not advised in this recipe because the liquid will be too thin.
G-F Testing Lab
For King Arthur Gluten-Free Multi-Purpose Flour: 12 ounces = 1 2/3 cups plus 1/2 cup
For Bob's Red Mill GF All-Purpose Baking Flour: 12 ounces = 2 1/4 cups plus 2 tablespoons
Note that waffles made with Bob's Red Mill will be somewhat darker and have a slight bean flavor.