Gallons of what? Different substances have different weights, so you need to be specific. If it's water, then you need to specify if it's salt water or fresh water, since they don't weigh the same.
That depends entirely on what is being measured because gallons is a measure of volume whereas pounds is a measure of weight. For example, 2.5 pounds of styrofoam will fill more gallons than would...
250 gallons would make 2000 pounds. Pounds is a unit of mass/weight. Gallons is a liquid measure. They do not convert cleanly. The density of the liquid being measured can make a big difference. If...