Anything! And not just games, you can also make screensavers, word processing applications, webpage refreshers, internet browsers...literally ANYTHING.
The only resaon I said "Unless you are making an RPG" is because RPG Maker all ready has the base laid out of you, and Game Maker does not. RPG's are difficult to make in Game Maker because there are a lot of complex equations involved that RPG Maker usually would just do for you.
But if you are making a platformer, and FPS, a flight simulator, a racing game, a sports game, or anything else that ISN'T an rpg, this is your program.