The Gap feature, if not used for adaptive cruise is used for the Green Vehicle you see in the DIC when you have a vehicle in front of you. The Gap based on your setting would automatically calculate the distance you should be based on speed. If you are following "too closely" the green vehicle will turn amber in color. I believe you will have 3 settings from close, medium and far. (I do not remember if they use different terms.) This does adjust based on your speed. If you are going 35 mph the gap does not have to be as large as when you are traveling 75 mph even on the same setting.
This is all based on my knowledge from my 2014 Impala LTZ current truck doesn't have the fancy features. I am also going based on the assumption that did not change since. So i still could be wrong.