The gated decay threshold of a reverb is the threshold of a compressor / expander or of a noise gate that is applied to the reverb portion of the signal.
Thus, a reverb that has a gated decay will apply a compressor or a noise gate to the reverb portion of the signal. The original signal is left unchanged in the reverberated mix.
While standard reverbs decay somewhat smoothly, a reverb with gated decay can decay abruptly as when the reverberated sound falls under the threshold of an expander or a noise gate.
Since a compressor / expander can have many thresholds at various input levels and since the reverb itself could be irregular, the resulting reverberated signal could sound bizarre and sometimes eerie.