Noise gate will be automatically active, if certain effects have been applied to a track.
It is active for instruments that use Effects like distortion, overdrive, and amp simulation. See:
Preventing noise from effects in GarageBand - Apple Support
Disturbed by unwanted noise in silent passages of your Real Instrument recordings? No problem—meet GarageBand's noise gate.
Effects like distortion, overdrive, and amp simulators involve amplification of the Real Instrument's input signal. This amplification may also boost otherwise unnoticeable instrument and background noises. The noise gate can prevent these unwanted, low-level sounds from entering the effects
To reduce noise in a silent passage:
- Select the desired track.
- Click the headphone icon to solo the track.
- Open the Track Info window.
- Expand the window so you can see "details".
- Click the Gate checkbox.
- Start playing the song.
- Adjust the Gate slider so that noise disappears from silent passages but the instrument still plays normally elsewhere.
I cannot enable it at all for vocal tracks, but for most guitar tracks and bass tracks it is already active, as soon as I create the track.
