A gallon of what? A US liquid gallon is 231 cubic inches or approximately 0.134 cubic feet. Which is to say approximately 7.48 gallons per cubic foot.
So presuming you want to fill the space with liquid, your space having a volume of 5 times 6 times 0.5 = 15 cubic feet, you would need 15 times 7.48 which is approximately 112.2 gallons.
If you are filling the space with dry (aka corn) gallons, 268.8025 cubic inches, or 6.43 gallons per cubic foot, you only need about 96.5 gallons -- give or take.