Spray a 10-inch tube or a 9x4x3 baking pan with baking spray.
Combine grapefruit juice, zest, oil, powdered sugar, and salt in a medium bowl and whisk quickly until glaze is smooth. If the glaze is too thick, add a little grapefruit juice, if it is too thin, add a little more juice. Drizzle glaze over the cake and let the glaze dry completely.