The military has plenty of anti-jamming technology don't they? I'm guessing if they wanted to do this right there would be ways that are a lot more sturdy than something a simple RF jammer can stop.
Like what? SNR-boosting encoding tricks to get around high-power noise are not going to be fast like you want with a gun. The form factor needs to be omnidirectional. You don't have high power available in a portable device. You can't just change frequencies, because an attacker is likely broadcasting high-bandwidth noise. And you don't know whether the user needs it to fail safe (the bad guys got the gun) or fail hot (the good guys need to use the gun). After that, physics doesn't leave a lot of other options for technology to fix anything!
The only effective "anti-jamming" technology is a radar/EM homing missile or some sort of artillery fired smart-shell acting on the same principle. There are technologies which work around specific functions of electronic warfare equipment but they are only effective in specific scenarios. There is no way to defeat a nearby spark gap generator without disabling the source itself.
It's much easier to attack such a system than to defend it. Attacker only needs a short window of time, so can emit a lot of power. "Smart" gun can't do that, it has to work for years on small battery, so its signal must be weaker by many orders of magnitude.
