I hope Elizabeth will be confessing to whatever she has done with her baby. Whether it's giving him away or killing him, I don't know. But, I don't think the Smiths have knowledge of what she did.
I guess that puts me in the minority here. I think the most that Tammi has to do with his disappearance is influencing EJ to do whatever she must to keep Gabriel away from Logan. I don't think Tammi is smart enough to run a blackmarket for babies. I do believe the way she tried to get Gabriel for her own last month was pretty horrid. If the Smiths did arrange for someone to take Gabriel why would EJ sit in jail to protect people she hardly knows?
Regarding the GPS, for all we know, LE already has it. They have not released an inventory of the car, the hotel rooms or EJ's personal possessions when arrested. Unless she had programmed in street addresses or locations I don't see how it's going to help much. Maybe LE got lucky though and found an address entered into it labeled "underground adoption society." Plus, you can not track someone's every move from a Garmin as far as I know. I'd be willing to bet they got more info from her cell phone than the GPS.
Sadly, all evidence so far points to Elizabeth murdering Gabriel. I hope and pray it's not true. It's obvious she didn't want him and she did not want Logan to have him. She wanted him to vanish from her life altogether so hopefully she did just give him away. But, she has a history of violence, anger problems, neglect and deception. She texted Logan and said she killed their baby and dumped him in the garbage. No innocent mother would ever say that, no matter how much she wanted to shock or hurt someone. Listen carefully to her jailhouse phone interview. Everytime she is asked about Gabriel, such as, "do you think he is safe" or "do you know if he is alive", she will pause or stumble over her words. You can tell when she is being deceptive because she will ask a question back to stall while she thinks of an answer, like "what do you mean?" Or she will say "beeeeecaaause, I trusted them" really drawing out the first word to stall.
IMO, EJ is for sure guilty of whatever evil thing has happened to Gabriel. The Smiths are guilty of bad judgement, arrogance, lack of intelligence, manipulation, but not evil.