Place the eggplant in a strainer. Sprinkle with salt. Let stand for 5 minutes. Using your hands, press out any excess liquid.
In a large Dutch oven or deep cast-iron skillet, heat the oil, three turns of the pan, over medium-high. Add the zucchini, peppers, onion, garlic, and thyme; season with salt and pepper. Cook, stirring occasionally, until the vegetables begin to soften, about 5 minutes. Add the eggplant. Cook until the vegetables are tender, 7 to 8 minutes more. Add the wine. Cook, stirring often, until absorbed, about 2 minutes. Add the tomatoes with juices and the basil. Let the ratatouille simmer over medium-low so the flavors can meld while you make the garlic bread.
Preheat the broiler. In a small saucepan, cook the butter, oil, and garlic over medium-high heat, stirring often, until the butter is infused with the garlic, about 2 minutes. On a rimmed baking sheet, arrange the bread in an even layer. Broil until lightly browned, about 2 minutes per side. Liberally brush the cut sides of the bread with the garlic butter. Top with the parm and parsley. Broil until lightly browned, about 4 minutes. Let cool for a minute. Cut into bite-sized pieces. Arrange in a 9-by-13-inch baking dish. Top with the ratatouille, then the Gruyere and Parm. Broil until brown, about 5 minutes.