Preheat oven to 400 degrees F with rack in middle.
Slice vegetables into 1/4-inch thick slices. In a large bowl, toss vegetables with cream, stock, tarragon and salt and pepper, garlic and Gorgonzola. Put into prepared gratin dish. Cover loosely with heavy duty foil. Bake for 1 hour, until vegetables are fork tender. If tender, remove foil, leave dish on middle rack and turn broiler on high. Broil gratin until top is golden, 3 to 4 minutes. Let cool 10 minutes before serving.
Gratin can be made 1 day ahead and reheated at 325 degrees F, covered with foil for approximately 50 minutes.