Which GNC are you going to? We need to know so one of our T-brothers can tackle you before you go in.
Just kidding. (But not really)
I think you'll find that the fat fast diet is several years old, and it has been somewhat replaced by the Velocity diet, which is pretty similar in nature. Either way you'll need:
A whole bunch of low-carb protein powder, as that's all you'll consume. Many people here (myself included) will tell you to go with the Low-Carb Grow! I can't gve you any other options as it's the only one I really like or use, but I can also afford to.
A big bag or two of milled flax seeds. That should be in a vaccum sealed bag with a re-sealable closure.
A whole bunch of fish oil capsules.
Some shaker cups.
That's it. By the way, everything but the Grow! can be bought in a decent grocery store or Cosco.