77.5151182665 according to http://www.convertunits.com/from/gallon/to/ounces. Are you looking for liquid ounces?
It really depends on what you are looking for.
128 fluid ounces in a gallon of water. 64 ounces is half.
If you are looking for volume gallon to mass ounces it will vary depending upon the density.
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If there are 4 quarts to 1 gallon, 4 quarts devided by 2 quarts = 2 quarts to 1 half gallon