I periodically look for a good (free) gate plugin like this, but I still haven't found a good one.
You might check out
Stormgate, although IIRC there's something about it I don't like--might be that there's no knob to change the length of all the gate spots in a pattern at once.
You could also try
Killergate+ (direct download link
here). I also forget what I didn't like about this one; maybe it didn't feel "snappy" or something?
Actually, you know what, here, look at the gates listed here:
clicky. It doesn't have Killergate+, but it has Stormgate, on of the other ones I was about to recommend, and a few others too. (I thought the download link was for all of them packaged together, but apparently it's just for "Super Trance Gate" or something.)
Another approach is to use a MIDI-controlled gate, like
DFX Midi Gater. I believe I've had better luck with that strategy.
Lastly, check out the free synth
TranceDrive. It has a neat little gate built into it. And it's pretty good for making trance sounds in general, as you might guess from the name.