Fire Nation Commander: "Hey, do you know where the Avatar is?"
Gyatso: "Oh, yeah... he's my nephew... he just ran away from home"
Fire Nation Commander: "You know... now would be a perfect time to get rid of these hippies"
— Why the hippies were all killed
"Thank god, that idiot left! I was getting sick of him and his "No uncle, drinking is illegal". "No uncle getting high is dangerous"..."
―Gyatso after Aang left

Gyatso was Aang's uncle, mentor, and legal guardian since Aang's dad had fled the country and Aang's mom was too high to give a crap about her son. Gyatso was responsible for training Aang and teaching him everything he knew, which pretty much included their sacred art, how to throw pies at people, how to fight "the Man" and how to get high properly. Gyatso's boss, leader of the High Council, alerted him his nephew was the Avatar. Gyatso said that they should take it nice and easy, but Aang overheard them and ran away on his flying hippo. Gyatso surprisingly did not care and went to get a drink. On his way he met some Fire Nation soldiers and they asked him where the Avatar was. He told them and continued to the bar. About 10 minutes later, the Fire Nation military had completely wiped out the Air Hippies.

Lack a sense of humor? Then get the hell out of here and go read Avatar Wiki's "real" article on Gyatso.