I am the author of the original Calf plugin, I had no idea it was being used in FFmpeg, great! The original plugin used to have a sidechain input to use as the level detection, but I am assuming the ported version just uses the input signal as this level.
The main parameter is the threshold, it sets the level below which gating begins to occur. Having 0 knee makes it a very sharp transition from signal to no signal, as well as the attack and release times. I have not actually used the FFmpeg ported version of this plugin before, but I assume it would be similar. It has quite a number of controls which probably makes it unsuitable to use without the usual GUI that shows the current level and the gating curve. One way to test it would be to install calf-plugins and play with the Calf Gate until you get a feel for the parameters, and then apply the same to FFmpeg. Sorry I cannot be more helpful because I did not write this port and I don't really understand the FFmpeg code.