First off, I need to know if you're confusing elite and world class. Guys like Floyd Mayweather were elite. Guys who aren't absolutely great fighters but who still fight at the highest level are all world class fighters. Of course Eubank beat world class fighters. Eubank was a world class fighter. If he wasn't, then neither were most of GG's opponents. Yes, Watson was beaten by Mike McCallum. But Mike McCallum was a great fighter. It doesn't mean that Watson wasn't world class. Seriously, pull GG's resume right now. Because if Benn, Eubank and Watson etc weren't world level fighters, then neither were the likes of: Macklin, Lemieux, Jacobs, Wade, Monroe, Rubio, Geale, Rosado and Murray etc.
For all of Eubanks faults one thing was for sure: The man was an iron man with an incredible chin and incredible durability. In a fight with unfortunate results(Watson 2) he was basically battered to the ground and ended up changing the outcome of the fight with one punch. GGG is going to have to be at his best for Eubank and if he is I think the main difference would be his jab which was a very good weapon. But Eubank had that incredibly awkward movement which GGG has never encountered so that might nullify GGGs jab. Have no idea who would win but it would be a very interesting fight.