There are usually only 3 options for any wheel that fits on a GMT400. Negative (lowrider) middle (stock or mild lift) positive x9.5 and x10 (big lift). Therefore depending on your application only one wheel will be appropriate.
actually it would be the opposite. not sure on these trucks what works best on a lowered truck, but a negative offset will push the wheel out away from the truck (best for lifted). a positive offset will pull the wheel in towards the truck which would cause all kinds of rubbing issues.
let me see if i can word this correctly to explain. lets say we have a 10" wide wheel. first we look at the center line of the wheel, which in this case would be 5" from the inside lip of the wheel. now we can determine offset which measures where the mounting surface of the wheel is in relation to the center line. if the wheel mounting surface is exactly 5" from the inside lip (right on the center line) then the wheel has a 0 offset. as we move the mounting surface towards the inside lip (which when mounted on the truck will push the tire away from the truck) the offset becomes negative. if we move the mounting surface towards the outer lip (pulling the tire in towards the truck) then the offset becomes positive. so if we have, say for instance, a wheel with a -6 offset, that means the the mounting surface is 6 mm closer to the inside lip of the wheel than the 0 offset wheel. thats why offset and backspacing are related. backspacing measures how far the mounting surface is from the inside lip. the same 0 offset wheel that is 10" wide would have a backspacing of 5". the tricky part is figuring out backspacing using the offset because offset is measure in mm where as backspacing is measured in inches. basically they both measure the same thing, just a different way of measuring. hopefully that helps!!!