water: 8 pounds per gallon
Gas: 6 pounds per gallon

there are some variable factors that come into play that effect the weight:
Specific gravity (density of the fluid)
Barometric pressure
air temperture
altitude

Volume:
1 gal(US) = 128 oz = 3.875 L = 387.541cc
1 oz = 2.957 cc

weight:
water:
1 oz = 1 oz
1 cc = 0.3382 oz

gas:
1 oz = 0.75 oz
1 cc = 0.2536 oz

550 cc (gas) = 139.48 oz = 8.7175 lbs = 3.9542 kg = 395.42 grams