Water is easy to remember. 1 litre weighs 1kg, 1 gallon weighs 10lb.
Fuel weighs about 0.72 times as much as water. So a gallon of fuel weighs a little over 7lb.
Rather embarrassingly, though, I can't remember whether this is an American or an Imperial gallon! I'm going to stick my neck out and say that it's an Imperial gallon.
(Exact weight would depend on the temperature, of course, which is why airliners measure fuel in terms of its weight rather than its volume.)
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