I think that JKD would have the perfect situation to gun defense if there was such a thing.
You see, JKD isn't a "style". Therefore, it doesn't have a composite set of gun defensive tactics. JKD uses whatever works.
Therefore, my JKD to a gun situation might be, giving the bad guy my wallet. Or, running perhaps. Because, imo, you just can't beat a bullet! The guy with the gun has the advantage and to think otherwise is just an exercise in foolishness and futility.
To someone who says that they know the perfect defense against guns, I'll read about you in the newspaper one day.
The best defense as with anything, is awareness. You must not put yourself in situations where you can easily be ambushed, etc..
If you're attacked in your home, that's another situation entirely. You have to do whatever you can. Actually, if you've no choice but to fight, one of the best choices is to rush the attacker before he's set. Don't give someone the chance to draw a bead on you.
I know this is difficult, but if you learn to read body language, perhaps attacking on the inhalation, just before the exhalation of breath (again by rushing) and using HKEs.
There is just no perfect defense against a gun.
By a cap gun or a toy blank gun and work those scenarios with a partner. See if you can hit your partner before you hear the "bang". You can practice this with ANY disarming techniques!