The documentation for the dsp::NoiseGate states that if the ratio is low enough it can be used as an expander, however, there is an assertion forcing the ratio to be above or equal to 1.0. This stops the gate being usable as an expander. I have created a pull request here to fix this and allow the noise gate to be used as an expander: https://github.com/juce-framework/JUCE/pull/1224