what does the syaing "Its so gay" mean

[Lilitu]

Generally here it means something someone has done which would be reminiscent of homosexual stereotypes or something overly sweet and sentimental.
E.G: "I want to bake rainbow cookies and pies made of sunshine and happiness." "Wow that's so gay."

Personally I don't find it offensive, probably because I'm sort of desensitised to it.