a gate is basically the same thing as an expander. the only difference is that a gate has an infinite ratio - it either opens or closes completely. an expander on the other hand can have other ratios, much softer ones.
an expander is the opposite of a compressor, a gate the opposite of a limiter.
a gate is a specialized expander (with a fixed, infinite ratio), like a limiter is a specialized compressor (also with a fixed, infinite ratio).
the difference is the same between a compressor and limiter , in this case , ull reduce the volume of a sound below the threshold , the expander will let some of the signal pass thru depending of the ratio u set , in the other hand the gate will stop any signal below the threashold .