What I did after I kind of felt I "mastered" GML and, mostly, the programming concepts in general, is I went away and spent a year on another engine, which shall not be named here, to learn to do c#, 3D stuff and finally be able to try a real-time debugger (that I miss a lot, lol).
Once you do that, you realise the principles themselves are all the same and that the language itself is just a means to an end, really.
Learn the language you want to use and that you're most comfortable with. Just dont think learning C# or whatever is the magic potion for anything, because it's really not.
If you like Game Maker, I'd say learn the heck out of GML. It'It's powerful enough for all your needs atm, I'm sure.