There's probably still a bootleg or 2 up on youtube if you take a look.
It's basically a combination of the Disney movie, the original novel, the German musical based on the Disney movie, and some original elements. It's darker and more down to Earth. Death, lust, and even a boner reference. When they performed the musical in New Jersey the theater had a note that it was not appropriate for people under 13.
Frollo's younger brother runs off with a gypsy woman after getting kicked out of the church. He gets sick and before he dies, he summons Frollo to his side and asks him to take care of his son. His son is Quasimodo, a deformed baby. Frollo goes to drown the child but then his a religious epiphany, that he could raise the child to be good and godly and to "think like him," and it's presented as Frollo thinking he has a chance to make up for his "sin" of not keeping his younger brother in God's graces. Quasimodo being Frollo's nephew adds an interesting dynamic.
Phoebus is still a war veteran but they bring up his horrific experiences in the war as a reason for his jaded behavior.
Quasimodo is mostly deaf, some productions have him being fully deaf though.
The gargoyles are there, but they aren't magical or really alive. They're figments of Quasimodo's imagination which act as surrogate companions as well as representations of his personal beliefs and conscious.
Esmeralda dies after being rescued. Frollo comes to Quasimodo and says they're finally free, and an enraged Quasimodo picks Frollo up and throws him from the bell tower. I'm not sure if this is in the licensed script anymore, but, at least in the last version with the premiere cast, the narration explains that they found Quasimodo's body years later holding Esmeralda's skeleton.