Preheat the oven to 375˚F. Thaw the pie shell if it's frozen.
Sprinkle a layer of mozzarella cheese on the bottom of the pie shell. Slice the tomatoes ¼" thick. Add a layer of sliced tomatoes on top of the cheese. Spread them with 1 tablespoon mayonnaise. Add a layer of Gruyere, a sprinkling of dried oregano, salt and pepper, a second layer of tomatoes. Spread with the final tablespoon of mayonnaise, a dusting of oregano, salt and pepper, and half of the remaining mozzarella cheese. Top with the final layer of tomatoes, oregano, salt and pepper, and the remaining mozzarella.
Bake for 30 - 40 minutes, or until the crust is lightly browned, the cheeses are melted, and the tomatoes are soft.