G letter, 0 number. Tis' a stepping, Intel calls it an S-step. Basically when a processor undergoes a change, the "S-step" changes, Which part that changes (letter, number) is based on the size/importance of the change, a large change results in the letter being changed, going up the alphabet, a small change results in the number changing.
The G0 stepping is supposedly a better chips at overclocking, I believe it also is cooler, smaller fab die? I dunno, some other people here can probably tell you what the main differences are, i never took much note of them.