Also depends on the brand. ISP Decimators are awesome in this regard, especially the G String II.
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Even the original ISP Decimator was awesome. I can only imagine how much more they improved since then. I had it in as first in line in my rack system to cut down the high gain hiss. I had it in a looper so it was not in the signal chain when not needed. It worked so well, it was as like it wasn't there at all except you the high gain hiss/noise is drastically reduced. There was no cutting of sustain at all. Anyway, back to the topic, it depends on the noise gate and how it is used. As mentioned, ISP makes an excellent one. Also as mentioned, depends on the threshold setting also. But if you have to crank the threshold knob to the point of cutting your sustain just to kill the noise, you might have other noise issues that you need to address first. My friend had a Boss NS-2. No matter what we tried, we had to turn up the threshold to kill the noise in his rig. But then his sustain suffered. Instead of trailing off naturally, it would cut off at the end. Then we decided to swap out the V1 preamp tube and bam... noise was brought down.
