A gate reduces the signal volume to zero when it goes below a certain threshold. If you're playing quiet passages you might need to kick it off, but then again, in quiet parts you probably won't be using massive ammounts of gain to dirty up the signal.

It sounds really patronising, but the best way to reduce noise is to not let it get in there in the first place. Try and experiment with your effects and the PSU's to see if there are any that cause more noise. If you power them all on batteries (as a test) you can also isolate if your psu is bringing noise into the chain. Loopers are also quite good to use, you can put the noisy pedals on a separate loop, so that they don't affect your signal when you don't need to use them.