That depends on the type material you are referring to. Also, these are two different types of measurements. Gallons is a measure of volume but the other is weight or mass. Please edit your question to include more context or details.
5 pounds
eight and a half pounds
2
Pounds of what? Feathers, lead, milk . . . .?
1 gallon of water weighs 8.3333 pounds. However, the weight depends upon the specific gravity of the liquid you are measuring.
how many pounds in a gallon
How many pounds of pecans do I have in a 55 gallon trash can
For plain water about 41.7 pounds at 8.345lbs per gallon.
8.6
At current prices, you would get around a half a gallon.
there are eight half-pints in a half-gallon
You can use 4 pounds as a rough measure. 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 the liquid is water, there is 8 pounds to a gallon.