well, compressors and gates primarily don't work on certain frequencies, they work on the whole spectrum.
There special ones (multiband compressors) that can divide the spectrum. I don't know any multiband gate, but maybe there are.
You can also make a poor's man multiband dynamic procesor (gate, limiter, compressor, expander) by dividing the frequencies with an equalizer or a bandpass filter, and then placing the plugin after that. That way it'll work on the frequency tha you dial on the filter.
You can also use a a filtered sidechain signal to trigger the processor. I thing de-essers work this way.
One nice trick with gates to fatten kicks: put a signal generator with a sine wave of low frequency behind a gate. Then trigger the gate with the original kick (you can use a filtered sidechain for that). That way both will sound in sync and it will fatten up the original.
And I don't think there is a difference in how realtime and nonrealitme processors work. Although there are some tricks you can play, for example you can copy your audio and shift it earlier and use it as the sidechain or trigger of the processor. That way you can simulate a lookahead effect, if your processor doesn't have it.
CHeers, Andrès