Only used here occasionally but, from the manual, page 60:
When set to GATE the Machinedrum pattern will start to play once receiving a MIDI NOTE ON command and stop when receiving a MIDI NOTE OFF command. This basically means that a pattern will play for as long as a key is being held on the external trigging device.
So, basically you could use the MD like a synth module that has up to 16 layers playing (synthesis, samples, or external input signal) on each incoming MIDI Note, and have the sound be completely different for each MIDI Note…
That’s a bit different to the MD’s feature.