GML, or Game Maker Language, is a a scripting language used with the program Game Maker. With the proper knowledge and commands, it can be used to set gravity, changs jump height/duration, set the controls, change the music, and much more. It's not very complicated at all to make a basic platformer using GML, as long as you have the proper knowledge. For aspiring game designers, it is a good start to learning more advanced scripting languages.
[I'm not incredibly experienced with GML, I just wrote this to inform those who haven't heard of it.]