GameMaker: Studio has a godawful room (level) editor, so with this simple tool, you can draw your room's terrain in your favourite image editor (could even be MS Paint) and then convert it into GamerMaker: Studio room file format.
For information on how to use the program, look at the "Distribution" folder.
For a precompiled binary (executable) ready for use on a Windows machine, also look at the "Distribution" folder. It also includes one Header file and one Footer file if you don't want to make your own (but you should).