A US liquid gallon is defined as 3.785411784 liters, the US dry gallon is 4.40488377086 liters, while the imperial gallon is 4.54609 liters. And that's just the currently used definitions...
If I ask Google for "1 gallon in liters", it assumes Imperial gallons which would account for a good chunk of the error.
Thank you for the correction. I should have done my math a bit better. I do appreciate this community for calling out incorrect data. One of the very few communities left that call errors out in a thought out way.
A gallon is 3.785 litres, and the euro is roughly $1.05.
So 2 euros per litre = one gallon costs $8.