Would expect 25% score roughly, so maybe 1 in 8, 1 in 10 if we say a lot of their expected score is from draws.
Can new GM beat someone in the top 100 in the world.

Why? I would say 10% because we are talking a player rated roughly 2500 winning against someone about 2650, which is a big difference at that level in my opinion. Majority of the play would be draws though, but I think that is obvious
What do you think is the chance of a player who just got his GM title,beat someone in the top 100 in the world,why?