Brush both sides of each cabbage wedge with olive oil. Sprinkle garlic powder, red pepper flakes, salt, and pepper over each wedge. Arrange wedges on a baking sheet.
Roast in the preheated oven for 15 minutes; flip cabbage and continue roasting until browned and charred in some areas, about 10 minutes more. Watch closely and remove from the oven when the cabbage is browned to perfection. Squeeze lemon over each wedge.