Hi Paul,
Side chaining usually means that another track is used as an additional input to influence the signal output of a compressor or a gate. The Channel Strip Noise Gate’s side chain however does not take another track’s signal, but it applies a filter to the track (or bus) it’s sitting on and uses this as the side chain’s input. In short, it copies its input, filters it, and uses this signal as the side chain, instead of another track. Proper side chaining where another track can be selected as input, is on the list of future updates.
Best,
Lars