Other than varying the "actors" and what they're wearing for the first minute or two of the feature, what other "plot line" are you expecting from the Gangbang Girl series?
Now that I think about it, this sounds like a good candidate for a reality series. No, really - behind the scenes of a porno movie. From development, through talent acquisition, "training", rehersals, filming, post-production, release (pun somewhat intended), any awards it might get, conventions...
I'd buy that for a dollar.