Re: These can't be DoSed, right?
Let's leave the gun part of it aside for a moment. This needs both WiFi and Cell to work (if ever). You can add the relevant information to cell broadcast in a mobile network and this will give you out-of-town coverage.
The problem is elsewhere - the gun nut lobby fav argument is that the gun is the means of defending against the big bad argument. This is supposedly, somehow, logical despite the government having drones, cruise missiles, stealth aircraft and being able to take you out on short order anywhere around the globe.
So any system that can give the government the keys to defining how weapons will be used get a "howler monkey troop" response regardless of its feasibility. This one is not feasible anyway as the means to determine "gun zone border" suggested in it are notoriously imprecise.
As far as the "gunsmith can change it" argument, that one is the wrong idea anyway. If an automated system for gun control is to be ever put in place it will have to be from scratch on all weapons and they will have to be manufactured and engineered so it is very difficult to remove. It is not a retrofit option - old guns will have to become illegal. As most of the Eu has proved it is easier to ban guns altogether.