Funny you ask, I was in a No Hollywood dispensary and I while I was getting a few clones the guy ask me if I had any G-C, I kinda looked at him funny then when he mentioned Green Crack I knew exactly what he was talking about. He said it is a very popular strain and he was having a hard time finding it lately.
Best info I can find is that it is a CA clone only strain and is a cross of Skunk #1 and a Cali Indica, or Skunk #1 X with Afgooy (or an Afgan strain) and then I have also found someone thinking it is the same strain as Mango. But the most credible, well most popular answer is the first cross I listed, said to be an old school skunk from the 80's and a dank old school cali indica. But so far, I have not found a definitive answer on this yet.
Other than the name it does sound like an interesting strain, will be on the look out to try some.