What a noise gate does, it switches of the audo when the level is below a given threshold.
the attack is how fast the audio will switch of after the signal is below the threshold.
hold is how long it stay closed, even when the signal is above the threshold.
Decay (Release) is the time from the signal is above the threshold to the gate is open again
For a voiceover, use a fast attack (1-5 ms) and a release around 100-300ms, start with hold = 0ms and turn it up if word endings get cut of.
It depend a lot on actual audio, is a noise gate is a useful tools
You can try exporting the audio and use a audio editor or DAW to get a better view of the audio and find the right values for Attack, Release, Hold.