A soft g, similar to the way gymnasium or giraffe is pronounced.
Also, the acronym was created to be a play on the peanut butter. So I believe it should be pronounced like the peanut butter.
To those of you defending your pronunciation because in the acronym the G stands for graphics, a word with a hard g, how do you pronounce CERN? The C stands for Conseil, so I imagine to be logically consistent you would say KERN?
In any case, I don't care about your logical consistency. Pronounce it however you like as long as a sizeable portion of the population pronounces it that way.
The only reason lightning never strikes in the same place twice, is that the same place isn't there the second time.