10 pounds weight
gallon is a measure of volume.one gallon of anything including water is still one gallon of that same thing.
One US gallon of water = about 3.785 liters One UK gallon of water = about 4.546 liters
If you mean hydrogen gas, H2, just naturally present in the water, then essentially none. If you mean how much hydrogen gas could be generated by the electrolysis of 1 gallon of water, that is a...
1000000 gallons 10^6
An Imperial gallon of water has a mass of 10 pounds.