I hope this one hasn't been here yet, but I've found an easy way to create gated reverb effect in caustic.
First you need your drumkit or instrument you want to use the effect on. Go to effects section and put there a reverb with quite long reverb time and high wet level. You can also add compressor or limiter if you want. Go to mixer and turn the volume of your instrument to zero.
Create a modular synth and put there machine input, envelope filter and 2 input mixer with modulator. It doesn't really matter which envelope you use. Select your instrument to machine input and connect it to the mixer. Connect the envelope to the mixers modulation input and connect the mixer to output.
You can either play your instrument trough the modular synth or use the synth just to open and close the gate. When you press any key on modular synth the gate is open. Play around with the envelopes releasing time if you want to have smoother effect. Try different settings for your reverb to find what works and try to add some noise to the mix.
I hope this helped someone. :)