Preheat oven 425 degrees. Line baking sheet with parchment paper (or foil, but be sure to spray with cooking spray if you use foil).
Roll dough into 14-inch circle on lightly floured counter; transfer to prepared sheet.
Melt butter in 12-inch nonstick pan over medium-high heat. Add mushrooms, leeks, and 1/2 teaspoon salt. Cook until beginning to brown, about 10 minutes. Stir in garlic and cook until fragrant, about 30 seconds.
Transfer vegetables to prepared dough, leaving 2-inch border around edge. Sprinkle with Gruyère and fold border over filling, pinching pleated dough to secure. Bake until crust is deep golden brown, 15 to 18 minutes, rotating halfway through baking. Cut into wedges and serve.