nope, you have to have the air from the body surrounding it on the stops. It goes like this: The lower notes have longer decays and sustain, and up the fretboard it is short, but still a decay, you can't use a gate (any) for those things. The one instrument that made me realise this was the MOOG guitar. Yes, the one with infinite endless POLYPHONIC sustain. Magnets in the fretwire or whatever. However, that guitar had a switch which reversed the endless sustain, and voila, all of a sudden that guitar sounded like a banjo. Without effects. Short stopped notes... and I though "what's that good for". Then the "aha" moment came up, and thought that "skip that polyphony thing, just go with the reverse sustain on bass!". THEN it occured to me that you should add something else on top too, the bass equivalent of a FISHMAN AURA pedal, but instead for acoustic guitar chambers sampled, as it is made for today, make it sampled upright basses bodies. It's a major chore, and too hard at the moment according to mr Fishman himself who's an accomplished upright player. I think the knick WIZMOON around here, is working on it at the moment, on another thread. I urged him to get a proper patent down first before going public.
Of course, a reverse switch should be used too, so you can soar with endless sustain solos on your fretless. Giving you lactic acid in the pinky fingerpads, and small biceps protruding out from the rest of the fingers, eventually. Nice workout at the fretboard gym.