You can't just insert Java into a game and allow people to script with it. You'd have to get an open-source scripting language like Lua or Ruby, but appeal for those tend to be torn. I personally don't like their formatting and one of the reasons I don't like them, so using one of them means I may have never learned how to script in Graal at all.
I'm backwards, I learned GS1 then GS2 then I learned programming proper, so I can't speak for everyone... but GS2 is much more structured then GS1 ever was. If you know any programming, you should be able to pick it up and jump in without a hitch.