Thank you all for your input. I myself didn't think the tracking would go over well, but it was an idea we threw around so I thought I should include it.
Our initial thought was to attach a sensor to your case/safe/cabinet and the alert would be from your safe being opened (not the movement/removal of the gun), but I think we got a little caught up in trying to do more. That seems to be the only angle that might make this idea work.
I understand that many households have security systems to alert you of break-ins, but that wouldn't apply to instances where one's child or their friends who are already in the house try to gain access to your safe.
Going with the idea of an alert for when your safe is opened, perhaps it could be targeted more towards gun-owning parents instead of gun owners in general? I understand the majority of gun owners already have multiple levels of security in place, but having a text alert once it's opened might be one more layer of peace of mind they might be willing to pay for.
By the way, our plan is that the app would be free with the one-time purchase of the sensor (don't know a price yet).