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