Preheat oven to 425° and line a rimmed baking pan with parchment paper; set aside. Heat 2 tbsp. oil in a large frying pan over medium heat. Add garlic clove and cook until lightly browned, about 2 minutes; remove garlic and reserve.
Pulse bread pieces in a food processor until coarse crumbs form. Add crumbs to frying pan (don't wipe out bowl of processor) with 1/4 tsp. each salt and pepper and cook, stirring, until crunchy and lightly golden, about 3 minutes. Remove from heat and set aside.
Put sage, reserved garlic clove, and lemon zest in food processor and pulse until well chopped. Add the pumpkin seeds and salt to taste and pulse until coarsely chopped.
Cut fish down the center to create 4 fillets. Rub each with scant 1 tsp. of remaining oil and season with salt and pepper. Place fillets on prepared baking pan and roast until fish starts to turn opaque, about 5 minutes.
Sprinkle bread crumbs over fish. Roast until crumbs are browned and fish is cooked through, about 5 minutes more. Transfer to plates, sprinkle with gremolata, and serve with lemon wedges and green salad.