Wash chard and trim discolored ends of stems. Cut out central stalk from leaves with a V-shaped incision that follows the tapering shape of the stalk. Thinly slice stalks and set aside. Chop leaves.
In a large frying pan over medium-high heat, melt 2 tablespoons butter. Add chard stalks and 1/2 teaspoon salt. Cook, stirring occasionally, until tender. Add chard leaves and garlic. Continue to cook, stirring occasionally, until leaves are tender. Stir in white pepper. Set aside and let cool slightly.
Butter a 9 x 12-inch baking dish. Spread cooked chard on bottom and sprinkle with ricotta salata. In a large bowl, whisk together eggs, cream, and remaining 1/2 teaspoon salt. Pour over chard and ricotta salata. Sprinkle with gruyere, cover with plastic wrap, and chill overnight.
Preheat oven to 350 degrees. Remove plastic wrap and bake until set and top is golden brown, 40 to 45 minutes. Serve hot, cut into squares. 6-8 servings.