The sub kick is making a continuous sound and I can't get it any shorter

If the sub kick is making a continuous sound, look in the Inspector window on the left side of the screen while selected on the Sub Kick track. If the Noise Gate plug-in is greyed-out, you will need to turn it on by pressing the power button that shows up when you hover your cursor over the plug-in.

If that is not the issue, you may have to open up the plug-in and adjust parameters. Although, if you haven't changed anything within the plug-in since installation, there shouldn't be any changes that need to be made.