Gating with no key filter is really really unlikely to work well IME.
A key filter lets you tailor the signal that the gate hears by cutting out the frequencies you dont want it to hear.
In your case you would want it to not listen to the top end hiss area at all, probably anything above 2KHz would be a fine to just ignore. That way you can have the gate close quickly as soon as any actual bass stops, and open quickly as soon as it starts. The level of hissy noise is not relevant to the gate action.
I would imagine that the Decimator, being based upon the HUSH noise reduction system would have all of this gubbins built in. The MXR smart gate has 3 modes (hiss, mid and full) which will give you an indication of the internal key filter set up that it uses.
As far as I can tell the NS-2 is either a full band gate or a full band expander, and ther eis no way to filter what it is listening to to determine when to open and close the gate.