It depends on where your kits/recipies come from.
Obviously, the US uses the US gallon, which equals 128 US ounces. To make it more confusing, US ounces are different than Imperial.
If using recipies or kits from the UK, you need to use Imperial gallons. 1 Imperial gallon = 1.2 US gallons.
Most recipes on this site will be in US gallons.