Using a gate will mute the noise when you're not speaking but it will still be there in the background when you do speak.
Regarding USB mics and Audition, there are a couple of things you can try.
First, in Windows go to your Device Manager, click on the USB port the mic is plugged into, go to the Drivers tab and the Update Driver via Web/Windows. This can fixt things sometimes.
Second, dowload Asio4all from
HERE.
Once it's installed, go to Edit/Audio Hardware Setup and select Asio4All in the driver window instead of whatever is presently selected.
Select "Control Panel" in the driver options and, hopefully, you will be able to select "USB Audio Device" under the WDM Device List.
Once you've done this, you should (well, may) be able to select your mic in the default input device box of the Audition/Edit/Audio Hardware setup menu.
Good luck--but USB mics are the work of the devil!
Bob