gates are origionally used after youve recorded a drum kit, you may have recorded a snare but its still catching some of the high hats in the recording, after tweaking you will be able to just hear the snare and the gate will have closed on the unwanted high hats.
you can also gate trance synths to get the rushy stutter you get in classic trance tunes!!
gates are used to control loads of different effect units and things aswell so reversing a gate can mean a few things, you could be reversing the gate pattern so instead of it going "do do doo" it would go "doo do do"!
you could also be reversing the gate open and close times!