Jgar, a buddy of mine has one of these JG antennas that he got to working. The antenna had been previously owned by two other guys before, and neither was able to get it to work good enough for them...for some reason.
I think the instructions may be vague and the owners are pretty much left to tuning from scratch. A tunable antenna that is adjustable over a wide range of frequencies, with little frequency related preset advice in the instructions, can be difficult. I recall they also got little to no help from the manufacture, which may be due to the legal issues already mentioned above and a new owner. You need to read the instructions very carefully, there may be more info there than you realize.
There is more to tuning a 5/8 wave, but some will tell you that this antenna is a simple matter of setting the correct lengths or maybe any length for the radiator and radials. Then all you have to do is simply adjust the gamma to the lowest SWR. That may be true, but that is not always as easy as these words would suggest.
Here is what I know about this antenna. The antenna elements are all pre-drilled and bolted together. So, the preset lengths for radials and radiator are set by construction and you don't have to measure. Exception is the top element is adjustable over a some range, maybe a foot or two.
The gamma match dog bone, the rod bracket that connects the gamma rod to the radiator, is to be placed between two small dents in the bottom element of the radiator, and the gamma rod is then adjusted up and down in the other end of this rod in order to set the match.
When you tune the gamma this close to the ground and elevate the antenna up higher, the match (SWR) is bound to change, so tuning at installed height is recommended, which is not always possible. By now you are seeing a little of why guys might have problems.
If you are able to set the match close to the ground, and after raising it you get a close match, then you might have to make a very small adjustment to the length to get the very best tune possible. At least try this first, before re-adjusting the gamma match and go the other way or pass up the sweet spot altogether. When you get close to a good match the steps to a perfect match are very small.
Tip: If you finally get a good match and won't to get it even closer to 1.0:1, I would tend to adjust the radiator tip element in <=1/8" steps going longer, since a gamma match is a capacitor and making the element longer is adding inductance. Gammas tend to tune an inductive (longer) element nicely.
Good luck and don't give up yet.