FK, I'm gonna offer my assessment of your situation, which may sound a bit harsh but it's 100% honest. I'll preface it by saying that my profession is in designing treasure hunting equipment and I have a Master's in EE.
First, the GPL device you have is a piece of junk. It cannot do any serious resistivity analysis and the meter markings are completely meaningless. I own an equivalent device from Accurate Locators (exact same design) and I've tested it. Would never use it to try and find anything except maybe a big shallow tunnel, and even then I wouldn't trust it. I've never heard of a single person using one of these devices that was ever successful with it.
Second, dowsing & LRLs don't work. Period. Dowsers & LRL users passionately disagree with me on this, yet no one has ever successfully shown me, in even the simplest test, that any of it works. And dowsers/LRL users generally have a pathetic track record. I've been studying this for 21 years and I own 40 or so LRLs plus a bunch of dowsing stuff. The vast majority of LRLs look like they were designed and built by a child, very amateurish. No serious metal detector or geophysics company messes with this stuff. In just 2 hours with me you could easily show me that I'm right.
Third, I don't know what you've been told about gravimetry but it cannot distinguish what causes gravity anomalies. If you were told that Enviroscan's measurements determined buried gold, then you were fed a big pile. Like resistivity, you can measure anomalies, but can't determine the cause of the anomalies.
In the few threads I've read about your endeavor, I'm not seeing any results that resemble legitimate. If that's all you had, I'd have to guess that likely nothing was there. I agree that the behavior of the FBI appears bizarre and possibly scandalous, but it also sounds like there's a whole lot of missing information. If you do manage to drag the FBI to court and you go armed with dowsing & LRL results, your day in court may end up being minutes in court. No one is going to take it seriously.
I suspect you'll dismiss my assessment and forge ahead. Serious consideration for what I'm telling you could be the best bargain in education you'll find anywhere. Ignoring it will also result in an education, but it'll be the painful way to get it.