So he's mixing:
1 gallon 2 quarts 3 pints of orange juice
1 gallon 3 quarts 7 pints of pineapple juice and
1 gallon 3 quarts 3 pints of ginger ale
that's:
3 gallons 8 quarts and 13 pints.
2 pints make a quart, so 13 pints make 6 quarts and 1 pint, so actually it's:
3 gallons 8+6=14 quarts and 1 pint.
4 quarts are a gallon, and 14 quarts are 3 gallons 2 quarts, so the final answer is:
3+3=6 gallons 2 quarts and 1 pint.