Potatoes: Put an oven rack in the upper third of the oven. Preheat the oven to 400 degrees F. Spray a heavy baking sheet with vegetable oil cooking spray. Put the potatoes and the olive oil in a medium bowl and toss until coated. Arrange the potatoes, cut side down, in a single layer on the prepared baking sheet and season with salt and pepper, to taste. Roast until golden, about 35 to 40 minutes Remove the potatoes from the oven and set aside to cool for 15 minutes.
Salad: Peel and trim the ends from each orange. Using a paring knife, cut along the membrane on both sides of each segment. Free the segments and add them to a salad bowl. Squeeze the juice from the membranes into a small bowl and reserve 1 tablespoon of juice for the dressing. Add the arugula, Gorgonzola, walnuts and the cooled potatoes to the salad bowl.
Dressing: In a small bowl, whisk together the olive oil, reserved orange juice, vinegar, and orange zest until smooth. Season with salt and pepper, to taste.
Add the dressing to the potato mixture and toss well. Season with salt and pepper, to taste, and serve.
To toast the walnuts, arrange them in a single layer on a baking sheet. Bake them in a preheated 350 degree F oven until lightly toasted, about 6 to 8 minutes. Cool completely before using.