Yes, I agree.... it does make it difficult to space anything.

But the mag usually is only one dose for most people daily.
(maybe two)

The mechanism of how magnesium and calcium inhibit absorption of gabapentin, is a solubility problem. Gabapentin is not well absorbed anyway...by itself.

Many drugs have his solubility issue with iron, calcium and magnesium. The minerals tend to attach to the drug molecule and make it too large to be absorbed. This is called "complexing" and binds both to each other making it less water soluble. Tetracyclines, Cipro (fluoroquinolones), also are complexed by calcium and magnesium, and get a warning sticker to remind the patients to avoid them when taking those antibiotics.

I don't see how applying to the skin would interfere much with gabapentin once it is absorbed and past the stomach, in the RDA dosing we use here

There is a doctor online on this link:
http://www.ncf-net.org/forum/neurontin98.htm
who makes a claim that magnesium interferes in the brain with Neurontin (gabapentin)....but I could not find corroboration of that yet.

Since magnesium is an essential mineral that we need each day in our food, it would be a serious, and I would think well documented situation, to have this interaction in the brain.

So sticking to using only the RDA of magnesium, which is what I suggest doesn't seem worrisome at this point. Really high dose magnesium that doctors give IV, that is another thing entirely.
When given this way, the doses are huge, and nothing like the doses we use here. Some migraine treatments use grams of magnesium intravenously.