Naw, I think Gary didn't do it that time ~ and most likely won't do it again, so there!
I'm biased towards the idea that a jail house confession is as good as anything ~
Certainly not when you have unreliable witnesses testifying to what Guandique might have told them in jail, and getting rewarded for their testimony in the form of a reduced sentence, perhaps. Even if this Guandique is a bum, and he certainly seems to be one, a "jailhouse confession" by itself doesn't mean much without other specific evidence linking him to Levy or to perhaps someone else who knew Levy.
I don't have access to the Guandique trial transcript, but if the prosecution had stronger evidence linking Guandique to the Levy murder, we would probably know about it via the media during the time of the trial.
In the meantime, you can probably presume that this upcoming hearing scheduled in the case deals with some piece of newly discovered evidence that might work toward exonerating Guandique in Levy's murder.