FWIW, I agree with the upthread opinions that we shouldn't do this (invent int64 GUCs). I don't think we need the added code bloat and risk of breaking user code that isn't expecting this new GUC type. We invented the notion of GUC units in part to ensure that int32 GUCs could be adapted to handle potentially-large numbers. And there's always the fallback position of using a float8 GUC if you really feel you need a wider range.
regards, tom lane
