SD-GATE 87 is a "real" sidechain gate/duck effect. As SD-COMP 87, it features 2 inputs pairs, but can also be used as a classic 1 stereo input + 1 stereo output effect while keeping the advantage of a "reference" section. In "Duck" mode, the gate is open when reference level goes below threshold (it works the opposite way). With "Tail" set to off, there is a good chance to get this gate open for a very short period once reference is fed again until it reaches Threshold value, producing an unwanted audio click. The "Tail" function helps you to get this gate opening only when reference level goes from high to low and not the opposite.
Whether you want to slightly polish a track or get your own special creative gate effects, SD-GATE 87 will do its job.