2Tb gluten to how much flour?
Wheat flour is rated as hard or soft depending on the gluten content. If your recipe calls for hard flour, you should use what is called "bread flour". It is much higher in gluten than all-purpose flour.
Cake flour has less gluten than bread flour, but it does contain a significant amount of gluten. While these estimates should not be taken as authoritative, cake flour contains roughly 6 - 8% protein (gluten) compared to about 11% in all-purpose flour, and about 14% in bread flour.
That is approximately 159.842 cups of flour
If you add wheat gluten to your recipe, maybe a tbsp or so (depending on how much flour? tbsp per 1 or 2 cups of flour?), that will serve as a substitute. Most often you can get good results with the substitution. Bread flour has more gluten and thus holds more CO2 from the yeast to make fluffier breads.
The softest flour to use in baking should be semolina flour. When adding liquid to the flour, it is best to not overmix as the gluten doesn't form too much to make the result dense.
Add one tablespoon of gluten for each cup of regular or all-purpose flour.
That is about 3.6 cups.
That is 7.5 cups of flour.
If I use 2 cups of flour from a 20 pound bag of flour how much is left
Either three of the 16oz bags of flour and you will have a cup and a half of flour left over or one of the 5lbs bags.
That is 1.6 cups