Position an oven rack in the middle of the oven and set the broiler to High.
Place the loaf of french bread, cut-side up, on a baking sheet. Broil 2 minutes, rotate the pan, then broil 2 more minutes, until the cut sides are slightly golden. Remove from the oven and let the bread cool while you cook the chicken.
Heat a large skillet over medium-high heat and add 2 tablespoons olive oil.
Toss the 1 1/2 pounds chicken pieces with 1/4 teaspoon salt and 1/4 teaspoon pepper. Add the chicken to the hot skillet and cook, stirring occasionally, until browned on all sides and cooked through, about 10–12 minutes.
Add the 2 minced garlic cloves to the skillet and stir for about 30 seconds. Pour in 2/3 cup buffalo wing sauce and add 1 tablespoon unsalted butter. Stir and cook 1–2 minutes more, tossing the chicken to coat in the sauce. Remove the skillet from the heat.
Return the toasted bread to the baking sheet, cut-sides up. Brush 1 1/2 tablespoons ranch dressing onto each cut side of the bread (total 3 tablespoons).
Evenly sprinkle about half of the 12 ounces grated fontina cheese over both bread halves. Divide the sauced chicken from the skillet evenly between the two bread halves, spooning the sauce as well.
Top each bread half with the remaining fontina and then evenly distribute the 6 ounces crumbled gorgonzola over both halves.
Place the baking sheet on the center rack and broil on High, watching closely and rotating the pan once or twice, until the cheese is melted and slightly golden and the bread is crisp, about 3–8 minutes depending on your broiler.
Remove from the oven. Drizzle additional buffalo wing sauce (reserve extra from your supply) over the breads as desired, then sprinkle with the 4 thinly sliced green onions and the chopped bunch of cilantro.
Cut the french breads into serving pieces and serve immediately.