Given A.I Harry would likely be wired into the internet, he probably figured out Norman was alive by tracking down assets and other projects that involved dealings with him during his European exile. Hell, AI Harry is based in Europe also, or maybe having Norman's brain waves as well as Harrys' somehow gave him that sort of second sight
Perhaps, the information came from Mendel Stromm?
Gaunt (Mendel Stromm) & Multivex Corporation encountered Spider-Man at around the same time than the events of 'Legacy of Evil' (April - June 1996).
Initially, the authors wanted to reveal that Gaunt was Harry Osborn but they changed the plan.
Harry Osborn died in 1993 and Mendel Stromm was thought to be dead from 1966 to 1996 (even though Gaunt was actually alive & working for Norman Osborn in Europe).
Still, in 1982, a robotic double of Mendel Stromm named the Robot-Master encountered Spider-Man.
The robotic double was apparently destroyed in 1982 but, in reality, in 2001, it was revealed that another version of the machine was still up & running.
A copy of Gaunt's brainwaves was continuously & automatically transferred into the new machine. The machine fully merged with Gaunt and became the current Robot-Master.
I assume that Gaunt might have built the new machine before 1993 and I speculate that it was this robotic double which contacted & worked with Harry Osborn before his death.
In this case, since the machine had a copy of Gaunt's memory, it also knew that Norman Osborn was still alive.
Obviously, the Osborn A.I. is based on the same technology than the Robot-Master.
Since Gaunt used Seward Trainer to spy on the Jackal, that's how the Robot-Master stole his technology.
Gabriel & Sarah might be Carrion variants.