Welcome, Mike! For better or worse, the subject happens to be very sensitive with respect to airguns. It's the policy of this and a few other forums to fly as far under the radar as possible in matters related to issues that could bring legal liability to the GTA or any of its members. As Bob (CDT) says, "Helps to keep us out of trouble".
The question of the legality of the unit that is built onto the Gamo Whisper, as well as other airguns, for the purpose of lowering decibels (even as little as 1 decibel) has to do with whether the unit can potentially be removed from the airgun, and mounted onto any firearm (and the law considers the potential in the broadest of interpretations). If the answer is yes, then it isn't legal to have in your possession without paying the necessary federal firearms fees, and you won't find it on a commercially available airgun in the US. Therefore, if a unit could be fitted to your Big Cat, it could probably be removed just as easily.
What you will find on those units that ARE available in the US, such as on the Whisper and others, is that they are integral to the gun, and cannot be removed with any reasonable expectation that they could be mounted onto a firearm. Here's some suggested reading:
http://tinyurl.com/332uneThis link will take you to Dr. Robert Beeman's website for a comprehensive treatise on the subject. It's a long read, but very informative.
Hopefully, that clears it up a little.
Bryan