You cannot really edit a game uploaded to the workshop. I would suggest commenting the workshop submission and ask the creator how they did something and if they would be willing to upload the source code. Many of the games uploaded are also recreations of various gamemaker tutorials and the creators should point you in the direction of said tutorials. There are also a few gamemaker games (such as spelunky) which have publicly avaliable source code.

Oh, ok then. Thank you for the responce (I don't know if I spelled that correctly).