I wrote this crate [1] as a compressor in Rust which is the opposite of a noise gate, as in gain reduction is applied after a threshold is passed instead of gain reduction applied if it is under a threshold.

If you want a really great approach to noise gating, a fixed threshold is fine but it works better when you apply it to the difference of two envelope followers - one with a short attack, long release (tracks input) and long attack, short release (tracks noise floor). Takes a bit to set it up, but it's a stupid simple way to get extremely effective gating and is easy to fine tune for your application. A lot of Voice Activity Detection (VAD) works this way; it's just a matter of tuning the coefficients and thresholds for your input.

Also useful reference for envelope following are the DAFX text [2], Will Pirkle's textbook on audio in C++ [3] and Zölzer's text [4]

[1] https://github.com/m-hilgendorf/rusty-compressor

[2] https://www.amazon.com/DAFX-Digital-Effects-Udo-Z%C3%B6lzer/...

[3] https://www.amazon.com/Designing-Audio-Effect-Plugins-C/dp/1...

[4] https://www.amazon.com/Digital-Audio-Signal-Processing-Z%C3%...

(pdfs can be found around the internet)