Hey,
What version are you using? Sonar 3 and 4 Producer editions have the Sonitus gate, which is excellent. Studio editions still have Cakewalk's FX 1 pack, which has a gate/expander in it. The only problem is they aren't installed by default. All you have to do is put in the install CD, start to regularly install the program, uncheck everything but audio effects when it asks you what to install, then click the box by "audio effects" and check "FX Dynamics processor" (or something around those lines). That should work for you.
Hope that made sense.