Nope. all attacks can be Gbed. the vulnerability window to do so varies based on the heavy speed. Generally speaking the slower the heavy the easier it is to GB them the faster the more you have to gb them on prediction. not reaction to the heavy.
While Raime is right, with lag involved it's a mess. I can confirm with video if I want to but it's a frigging crap shoot whether you actually pull off the gb during any attack. Don't rely on catching someone with it unless they are preoccupied or just changed your mix up.