Actually waves does have one, but it's got a kind of roundabout way of doing it. You have to make a stereo bus and route the track you're keying to the left channel and the track you're gating to the right channel. Then put the C1 Comp - Gate on the Stereo Bus and set the Key mode to "L -> R".
edit: When you do this, you'll notice that the track on the right on the right is gated with the one of the left as the key, however if you're gating snare or toms, for example, you don't want this track panned all the way to the right, so you have to use the stereo panner to pan the right channel to the center. Then you could bounce the file to disk and you have a mono, gated track, just like you wanted.
edit 2: If you're talking about EQ Sidechain, the Waves C1 has that too. Just hit the button towards the bottom of the gate column (I can't remember what it says initially) until it says Sidechain and adjust the EQ on the bottom to your liking.
