Make sure that your release time is on a slow setting. A fast release time will sound unnatural and the beginning of phrases may not be present. The ends of words may also be cut off if your release time is too fast. Keep the rate set between one and 1.5 seconds.
Put an external equalizer in your recording chain after the gate. Set it to filter extremely low and extremely high noises, like bleed from a bass amp or rustling papers.
Record a test vocal using the setup you just created. Make sure the singer sings the way you want it to sound upon playback. If the singer does not put their full effort into the take, it will not help you properly set your noise gate.
If your vocals sound clipped or unnatural, you need to reduce the rate of the gate. If too much external noise is getting in, you need to increase the rate.
If there are noises getting in that seem to be on a different frequency from your vocals, use the equalizer to filter them out. Continue to make the noise while the singer is singing into the microphone. Listen to make sure you are not altering the tone of the singers voice in an unpleasant way.