How to find the difference between the current money, like in the simulators?

Obviously, in order for this to happen you probably have at least one function somewhere that adds money to the player's balance, right?

You can take a far simpler approach and, when you fire the RemoteEvent to the client to make the ScreenGui appear, just pass the amount of money added as an argument and go from there.

However, if for some odd reason you really want to do it the way you talked about, here's how I would do it:

local previousAmount = 1000 -- Variable holds the amount of money the player has before any transactions. I wrote 1000 as a starting amount

Money.Changed:Connect(function(newValue)
	print(newValue - previousAmount) -- This'll tell you how much was gained.
end)

The code I just wrote isn't perfect though and there's ways it can go wrong, which is why I suggest doing the first thing I talked about instead.