I believe tapioca is wheat free. Here is a definition I got from a Google search:
"Tapioca is basically a root starch derived from the cassava, or yuca plant. It's often used to thicken soups and sweeten the flavor of baked goods."
In this case, the tapioca is used to thicken the juices of the pie. I've used it in apple pies instead of flour. I don't have an exact calculation for this recipe, but maybe this will help:
...at Ocean Spray's site:
for 1 tablespoon cornstarch substitute 2 tablespoons flour or 1 1/3 tablespoons quick-cooking tapioca
for 1 tablespoon flour substitute 1/2 tablespoon cornstarch or 2 teaspoons quick-cooking tapioca
for 1 tablespoon tapioca substitute 1 1/2 tablespoons flour
I wouldn't get stressed about the exact calculations. I've found that the recipes are very forgiving. I'd guess that for 3 Tbl tapioca, 1 Tbl cornstarch should do it.