If we are only talking about offense, then you can say he is not an elite hall of fame offensive player, though he certainly is allstar calliber at the offensive end and makes other players better by stretching the floor and passing. Given that much of his game is jump shooting (though jump shooting at a very high percentage), he is not the type of player that can always take over at the end of a game, but there are very few of those (and we have two in Paul and Ray).
But KG is an elite hall of fame defender, both a great one-on-one defender and a great team defender. Further, he can defend a wide variety of players in the post and on the perimiter, which is very helpful for any team's defense. This makes KG an elite hall of fame player (or will when retires). This makes KG underated (not overrated).
Calling KG overrated because he is not a dominant scorer at the end of games is as silly as saying Bill Russell was overrated. KG is actually a much better offensive player than Russell was. And KG impacts the end of games in ways other than scoring (and he is not bad at the scoring part either).