A Peak Limiter sets the max volume that will be allowed for that instrument. It's nice when your cranking your home recordings in your car because you won't blow your ear drums or speakers on a loud snare hit
A Noise Gate trys to isolate a mic to only one instrument, like a snare drum, without picking up other instruments.
the noisegate works by setting a threshold, anything above that threshold , the gate stays open and lets the source signal through. Anything below the threshold, the gate closes and nothing comes through.