They cannot be separated by distillation, as the boiling point is so low. I'd suggest that you add 10% aqueous NaOH slowly until all the GBL present has been converted to NaGHB (solution pH 7-8), then you dilute this solution to twice its volume with water and wash out any remaining Ethoxydiglycol (it should be relativey inert to NaOH) by shaking repeatedly with a low-boiling non-polar solvent (diethyl ether or possibly dichloromethane), then finally you boil off 3/4 of the water to concentrate the GHB solution (and any remaining non-polar solvent will be carried away with it).
You probably do not need to worry about Ethoxydiglycol traces afterwards, as at least its toxicity seems to be fairly low: Diethylene Glycol Monoethyl Ether (Ethoxydiglycol ) Chemical Profile (http://www.intox.org/databank/documents/chemical/diglmee/cie65.htm)
(http://www.intox.org/databank/documents/chemical/diglmee/cie65.htm)