Place pizza stone or inverted baking sheet in oven and heat to 500 degrees.
Heat 1 tablespoon oil in a large skillet over medium heat. Cook garlic until just tender, about 1 minute. Add chard stems and cook, stirring, until tender, about 5 minutes. Working in batches, add leaves and cook until just wilted, about 5 minutes. Season with salt and set aside.
Stretch dough into a 10-by-14-inch rectangle and transfer to a piece of parchment. Brush with remaining oil and season with salt. Sprinkle with half the Gruyere, then top with chard and sprinkle with remaining Gruyere.
Transfer to the oven: Slide parchment onto stone or baking sheet and bake pizza until cheese is bubbly and crust is golden, 13 to 15 minutes. Sprinkle with pepper flakes.
The Heat Is On
Ceramic pizza stones pull moisture from the exterior of the dough for a perfectly crisp crust. A preheated baking sheet works almost as well.