I've heard mixed things about KG off the court. He's supposedly a bully to ballboys, and he's apparently been pretty rude to several fans who have asked for autographs. At the same time, I'm sure there are plenty of stories of him being a good guy off the court, as well.
Here's a link: http://forums.celticsblog.com/index.php?topic=18763.0
... and an anecdote from one of the aforementioned ballboys:
http://bbs.clutchfans.net/showthread.php?t=176871
I think that anyone who deals with KG right before or right after a game is going to have a different experience than someone who sees him in most other settings. Honestly, have you ever seen a player get more "in the zone"?
This is the problem with any of these accounts of a pro athlete, or famous person in general being a "jerk". When you see someone in certain circumstances, it is really tough to make a legit judgement on their character.
Maybe they were having a bad day. Maybe they were just zoned in getting ready for the game. Maybe they were having a nice meal with their loved ones when you decided to interrupt it (and were the 18th person to do so that night).
Also, having a warm, outgoing personality is not a prerequisite for being a pro athlete. Some people are just not as good at dealing with people.
My impression of KG is that he is a very thoughtful guy, very loyal to those he is close to, but someone who also keeps a strong wall up around him. He can be charming when he is in the right mood, but unless he gets ramped up for that, he comes off cold.
I don't think that makes him a bad guy. Just not someone I would want to grab a beer with.
...on the court on the other hand, he is a total jerk...and that is part of the reason he is so good.