The correct answer is: $128\text{ fl oz (US liquid)}$
Explanation
Most people asking this mean US liquid gallons and US fluid ounces. A US liquid gallon contains 4 quarts, each quart is 32 US fluid ounces, so $4\times32=128$ US fluid ounces. Note there is also an Imperial (UK) gallon, which equals 160 Imperial fluid ounces. Also, “oz” can mean a unit of weight — ounces by weight depend on the substance’s density (e.g., water).
Steps:
- US liquid gallon:
$$1\text{ gal (US)}=4\text{ qt},\quad 1\text{ qt}=32\text{ fl oz}$$
$$1\text{ gal (US)}=4\times32=128\text{ fl oz}$$
- Imperial (UK) gallon:
$$1\text{ gal (Imp)}=8\text{ pt},\quad 1\text{ pt}=20\text{ fl oz}$$
$$1\text{ gal (Imp)}=8\times20=160\text{ fl oz}$$
- Quick metric conversions:
$$1\text{ US fl oz}\approx29.5735295625\text{ mL}\quad\Rightarrow\quad1\text{ US gal}\approx3.785411784\text{ L}$$
$$1\text{ Imp fl oz}\approx28.4130625\text{ mL}\quad\Rightarrow\quad1\text{ Imp gal}\approx4.54609\text{ L}$$
Note: If you meant ounces by weight (avoirdupois) for a specific liquid, tell me which liquid and I can convert volume to mass for you.