The gate in a synthesizer controls the duration of the note you are playing. In the Nintendo KORG DS-10 title, the gate enables to shorten the duration of the sound being played at each beat of the loop (pattern).
The term LEGATO means that there is no gap between the notes being played, that is, the keyboard virtual keys are never actually depressed, it's one long continuous note modulating. The lower the gate (in terms of percentage), the shorter the duration of the sound. Playing around with the gate obviously enables to put rhythm into loops.
The (very short) video above shows the effects of the gate settings on the notes being played.