Water holds onto the GBL far better than xylene does, and better than DCM does... although I believe it's miscible in all three. Moral: don't use xylene to extract GBL from water--use ethyl acetate, DCM, etc.
I assume there's no surfactants in the mixture, if there is... give up hope. This is what I'd do with your mixture:
While stirring, titrate with 10% NaOH to a pH of 12. Dicard the organic layer. Then from there, acidify the aqueous with ~15% sulfuric acid until the pH is around 1 (go slow) and extract that with DCM (equal volume to the aqueous, but split it into 5 extractions... ie if it's 500ml aqueous, extract 5x100ml DCM), simple distill off the DCM with no vacuum and fractionally distill GBL under vacuum.
Good luck and let us know how it goes. If you have any luck with THF->GBL I'd love to hear it.