Inspect the gable vent and its seal with the side of the house both from inside the attic and from outside using a ladder. Look for any broken seals between the gable vent and the house and apply silicone caulk to any cracked or broken seals.
Trim identical triangle-shaped wood pieces from a solid sheet of wood. Design the triangles so the shortest side is about 6 inches long and the longest side is as long as the gable vent’s sides are tall. Nail the wedges onto the sides of the gable vent from inside the attic, with the 6-inch sides pointing upward.
Measure the distance between the triangle pieces you installed. Cut a piece of plywood that is the same width as the distance between the triangle pieces and the same height as the triangle pieces.
Place the plywood piece onto the edges of the triangle pieces and drive nails through the plywood and into the triangles. This allows for air flow but does not allow water to come into the attic.