I'm quite certain that if the GBI was as interested in finding that gun as they were in creating a show trial, that gun would be found, and it would be traced back to that piece of sh*t Arbery.
You may feel differently, but I think it's foolish to postulate more than one thief when we know this guy kept coming back.
You might be absolutely right that Arbery stole the gun. If that is the case, Travis can take the stand at his trial and explain to the court why he believes that Arbery stole the gun.