This could have been a great opportunity to get most or all of the drug thugs, by mounting a tiny GPS tracking device in each weapon and targeting accordingly.
Not really. We don't know where most of the guns ended up. Most likely a lot of them ended up in innocent hands, people who are simply trying to survive in what is rapidly becoming a failed state.
I don't think any practical GPS trackers such as you posit actually exist. I am not saying that they are impossible, just highly unlikely to be both small enough, dependable enough, long lasting enough and cheap enough to make sense in such an application.
10 years from now, maybe. I don't think we are there yet.