MIDI Gater is a simple, handy, MIDI-controlled audio gate. The idea is that, when you play a MIDI note, audio turns on. When you release the note, audio turns off.
MIDI Gater is a little bit unusual as far as MIDI controlled gates go, though, because it is actually kind of "polyphonic." What that means is that, the more notes you play, the louder your audio signal gets. Basically each note turns on a copy of your audio signal. The volume of each copy is determined by the key velocity of the MIDI note. If you have the velocity influence parameter turned up to 100%, then the volume of each copy directly corresponds to the MIDI note velocity. If you turn velocity influence down to 0%, then all notes result in copies of the audio playing back at full volume. Values of velocity influence between 0% and 100% result in the key velocity having more or less effect on the audio volume.
If the audio simply turned on as soon as you played a MIDI note, you would hear clicky noises every time you played or released a note. This is caused by glitchy digital waveform discontinuities. In order to avoid this, MIDI Gater fades the volume in when notes start and fades it out when notes stop. You can control the duration of these fades with the attack and release parameters. Attack controls the duration of the fade-in slope when you begin playing a note, and release controls the duration of the fade-out slope when you stop playing a note.
If you don't want the sound to turn completely off when no notes are playing, then you can adjust the floor parameter to control the baseline volume level of input audio that will always be present. Any increase in the floor level subtracts from the maximum volume level of a single note; that way the sum audio level of a sustained maximum-velocity note will still be unity gain of the input audio. This also means that maximum floor will cause notes to have no effect.
The gate on/off transitions can be made by ramping volume up and down, or by opening and closing a low-pass filter (which is more like how sound naturally decays). You can select between these options using the gate mode parameter. Low-pass gate mode uses a little more processing power than amplitude.