yeah i think corbzy has it right. a gate is basically a gate that lets certain noise out. or a certain amount of noise. from what i can remember/understand its basically something that cuts off the output of sound once the audio falls below a set parameter. i BELIEVE (but im probably wrong) that it an work the other way too, where it shuts off audio once the signal gets loud enough to a set parameter. it basically only lets a certain amount of noise out (which you determine).
as for a reversed gate i have no idea. ive been trying to figure this out for the past couple days.

someone PLEASE correct me if im wrong!!!

-hiro