There's a lot of conjecture regarding Gabapentin administration. One thing we do seem to know for certain, is that the mechanism which Gabapentin uses to enter circulation through the proximal small bowel has a limited capacity and can be saturated to the point that no more Gabapentin can be absorbed. I've read two different sources that claim the maximum dose that could be aborbed in a given day by a human with normal bowel motility would be about 5,000mg.
Anyway, I am not against taking the 600mg as a single dosel, but my understanding was that no more than about 300g-400mg can be absorbed at any given time, so folks commonly will take 300mg -> 45 mins later ->300mg as opposed to the 600mg at once. My personal experience tells me that this is pretty close to accurate. I used to be so stupid with Gabapentin. I would take 25 600mg tablets in a day for instance. It was a total waste and I regret not trying to understand better what was happening with the drug.
In the end, I would get significantly better effects by taking 3,000mg in a day then I would 12,000mg in a day or what have you. These dosages are all on the high side, but I became acquainted with Gabapentin originally for its ability to stop my legs kicking in withdrawal.
