Combine 2 1/2 cups broth and 1 cup water in a small saucepan. Bring to a boil. Slowly whisk in cornmeal and pepper until smooth. Reduce heat to low, cover and cook, stirring occasionally, until very thick and no longer grainy, 10 to 15 minutes. Stir in Gorgonzola; remove the polenta from the heat.

Meanwhile, heat oil in a large nonstick skillet over medium-high heat. Add garlic and cook, stirring constantly, until fragrant, 30 seconds to 1 minute. Stir in zucchini and squash and cook, stirring occasionally, until starting to soften and brown in places, about 5 minutes. Sprinkle flour over the vegetables; stir to coat. Stir in the remaining 1 cup broth and bring to a boil, stirring often. Reduce heat to medium-low and simmer, stirring occasionally, until thickened and the vegetables are tender, 1 to 3 minutes. Stir in basil; serve the saute over the polenta.