If JG had an OL, then his completion % would probably not be as high and therefore you would probably not think so highly of him. Most of his passes are short dump passes due to the OL not being any good. Hard to get those picked off and easy to complete but VERY difficult to score any points when that is your passing game.
JG is a slightly below average QB in the SEC and if anyone behind him was not a freshman or a grad transfer then he would not have been the starter. He has not shown any progression in 2 years of reading blitzes, checking out of a bad play, or getting rid of the ball quicker, that is on him. Yeah he has had a few different head coaches, OC's, and such but so have the other players and most of them have shown improvement.
Lik eeinstein said, doing the same thing over and over and expecting different results is an act of lunacy. sticking with JG and expecting that all of the sudden he is going to become a really top of the pack QB in this league is insane. He will continue to get slightly below average results. His rating out of high school was overstated mainly because his faults are easy to cover up in high school. Camps do not focus on the management of the game, just the physical side of being a QB, hence why he was rated so high.