Preheat the oven to 375°F and lightly butter a medium baking dish.
Melt the butter in a saucepan over medium heat. Add the minced garlic and cook for 1 minute until fragrant.
Stir in the heavy cream, salt, and black pepper. Heat until the mixture just begins to simmer, then remove from the heat.
Arrange half the squash slices in the baking dish. Pour half the cream mixture over the top and sprinkle with half the Gruyere cheese. Repeat with the remaining squash, cream mixture, and cheese.
In a mixing bowl, combine the breadcrumbs, chopped sage, and olive oil. Toss until the crumbs are evenly coated.
Sprinkle the breadcrumb mixture evenly over the top of the gratin.
Bake for 40 to 45 minutes until the squash is tender and the topping is golden brown.