When Buffy The Vampire Slayer was on people often referred to her friends as the Scooby Gang, or Scoobies. Now SMG is actually in a new movie about the cartoon characters. So what’s up with that? Why did people call the BVS characters the Scoobies? Was SMG in some earlier version of ScoobyDoo, or the TV show?
I always assumed they called themselves the Scoobies because they were a gang of teenagers chasing after monsters, but maybe there’s some deep metaphysical link I’m missing. 
To the best of my knowledge, Gellar wasn’t in any Scooby Doo related materials prior to her movie appearance. Scooby Doo first aired in 1969; Gellar was born 1977. On Buffy, they called themselves the Scooby gang because, like the teenagers on Scooby Doo, they solved mysteries and fought evil.