I haven't tried to go back to the Spellhold Island after traveling through the underdark, but I suppose it is possible, so just try it out!
Go to the City Gates, click on Brynnlaw or the Asylum and see if you can travel there. If you can, then you'll probably also be able to go back into the maze and retreive the G-String (er...Gesen-String, that is ~lol~)
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