My only advice is that its a waste of money.
There is another forum on Med Help, called Weight Loss Alternatives, where people discuss things like this. I'm sure they could help you over there.
Yes, we would help you over there - but we can help you here too! B12 shots are not a waste of time, but they are not some miracle answer either. They increase your metabolism slightly and give you some extra energy. I've taken them and definitely felt a good boost, but I don't take them anymore. I've found a supplement works well too. Specifically, I haven't tried GNC, so I have no info on that. The main thing is, once you take it - DO SOMETHING like workout with the extra energy or it goes to waste. Good Luck!
I took them for about a month. Can't say they helped with weight loss, but they did help with energy. In the end, it was a waste of money and a pain in the behind to get them.
There are better methods for effective weight loss.