Unless you have a really poor diet, Mg withdrawal shouldn't be a problem, most people should ingest at least some Mg in their diet. ((e.g. 30g of peanuts has approx. 50mg of Mg in it, bran cereal has up to 150mg/cup, the RDA is 400mg of Mg for a normal healthy adult) MgO is likely converted to magnesium chloride in the acid in your stomach. 500mg of oxide may seem a lot but again, that's still under the RDA for Mg (MgO is not 100% Mg by weight) ... the worst that will happen is you will get runny shits if you're taking too much anyway.
If it makes you feel any better, AFAIK gabapentin does not "deplete" magnesium stores - it is not a Mg releasing agent, it instead acts on voltage gated calcium channels (again, not a releasing agent) and possibly also alters GABA biosynthesis/metabolism.
Are you taking any other "supplements"? Magnesium isn't the kind of thing I would expect to have a withdrawal from, but there's lots of other stuff out there that is not anywhere near as benign.