How may quarts equal a gallon?

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.