I helped design that gate. The "key" is really a trigger input. TRIGGER didn't fit on he panel. I was trained on analog synths way back and in the mid 1980s realized there was not an affordable gate with an external trigger, something I used a lot during my days working in studios as an engineer.
With the key an audio source you don't hear on the track could control a mono or stereo signal. Or you could Y an audio signal to trigger whatever was going thru the gate. Or you could use an eq on the Yd input signal to fine tune when the gate opened.
I was big on using indirectly related audio material as a key source. So something more or less continuous would be chopped up.
The key monitor button was for listening to and tweaking the eq.
A side chain is simply the non audio control signal in a processor. They all have a side chain but only some let you interrupt the side chain, taking the output elsewhere or what have you.
So a key iinput really doesn't interrupt the side chain, it allows a different signal at th input to the control side of the unit. It was done this way to keep costs down.
Hope this helps.