I'm not going to argue that Game Maker is a true engine, but I will also say this:
The capabilities of Game Maker are endless, and it does have it's own language to learn that is based heavily on C. Sure, on the outside it is designed to be usable by any and all, but if you dive deeper into it, it can actually get a lot done.
Now, aside for the praise I just gave it, it is a small time developing program, and using it isn't a guarantee to becoming a better game designer. Though, don't knock it just because it's simple. I wish all high end game engines worked like it did, while we'd also have an excess of stuffty games, we could also potentially have an excess of amazing games.
Besides, a lot of games were made on Game Maker that you probably didn't know had been.