...I would have said something, probalby rude, to the GM.
Well then such "friendliness" doesn't seem very genuine. Apparently when people say hello they are really saying "You better talk to me and make me feel good or I'm going to be mean." Scary! A truly friendly person would say hello only in the hopes of making someone feel better, not expecting something in return. Nothing wrong with enjoying something in return of course, but that's a very different thing.
I find it unsettling when some GM's and other high level players are rather gruff with people. It's one thing to walk up and shove a microphone/video camera in their face, so I can understand not being happy with that, but it's another to simply say Hello and be totally unresponsive.
But I also know the type of personalities that this game attracts. And frankly I've found it rare to meet people who are very high level players with reasonable social skills. Many tend to be very introverted to the point of coming off very rude.
And everybody has a bad day. I usually pride myself on being accepting of people and atypical behavior, but every so often I can have a really bad day and get set-off by minor annoyances and turn into a total asshole and regret it within minutes.