I'm not sure exactly what you are asking. Certainly, there's no gun in FSX if, by "Gun", you mean something that shoots bullets at other planes according to some physic's laws, trying to put down those in flames...
If someone managed to write some kind of a gauge that makes noises when you pull the trigger and nothing else, I'd say that might be possible, but certainly it's not built into the base product, nor our MFD Gun page (is listed as INOP for a reason...) will never know anything about it...
