Gamemaker? you mean sort of like RPGmaker? then that means it wouldnt exactly have a normal set of code for porting, someone would need to clone it form scratch.
He hasn't coded this game in anything, it is done in a GameMaker. It is therefore impossible to port as there is no source code to speak of (nothing of any use anyway).
GameMaker uses its own scripting language called GML. But, only the behaviors for the objects are written in GML. Everything else is point and click. To port it you would have to rewrite the whole thing.