Donna,
While the GMT is a good gold nugget detector, it lacks everything you might what for coin, token, jewelry. and many relic hunting tasks because it doesn't have discrimination. So, scratch that one from your list. GMT
The DFX, even with the factory "Prospecting" program, isn't really meant to be a nugget hunting tool. Learning to program various changes with a DFX or XLT really isn't all that difficult, however, it does take a little time to learn what these models can do, as well as pick up on their weaknesses. Hitting ghost towns and older sites often means working in and around building rubble and/or brush. For best performance in these conditions or around trash, you're better served with a dedicated slow-sweep detector, and that's not the DFX. You can try to make it work that way, but it can't match a model that was designed to work that way from the start. Personally, I've owned a few DFX's and they never came close to the performance I got from an XLT or an MXT. I'd eliminate the DFX from consideration. DFX
That brings us to the MXT which is honestly one of the best detectors White's has ever produced. very versatile! A very good nugget hunting unit with a smaller coil. My favorite is the 5.3 Eclipse, but I've done OK with a 'Shooter' coil (too small for me most of the time), and if I wasn't using a 5.3 Eclipse (a 6