Preheat oven to 350 degrees. Place zucchini slices in large saucepan with water (enough to cover) and bring to a boil. Cook zucchini until tender (about 3-4 minutes) and remove from heat and drain. Place zucchini slices in greased casserole dish and sprinkle with garlic salt. Top with bread crumbs. Mix mayonnaise and sour cream together and spread evenly over bread crumbs (The mixture will stick to bread crumbs, but try to make it as even as possible). Pour tomatoes over mayonnaise/sour cream mixture. Sprinkle sugar over tomatoes and then sprinkle more garlic salt. Cover with cheese.
Cover dish loosely with foil and bake for 30 minutes or until casserole begins to bubble. Remove foil and bake for additional 5-10 minutes.