There are 128 fluid ounces in a US gallon. If you are referring to the UK (imperial) gallon, it contains 160 fluid ounces.

Summary:

Measurement System| Ounces per Gallon
---|---
US gallon| 128 fluid ounces
UK (imperial) gallon| 160 fluid ounces

Let me know if you need conversions for dry ounces or other measurements!