Hah! You think Real men program in C++?? real men write their own program languages + compilers, none of this pre-made language nonsense. Infact.. Real men build their own CPUs with custom instruction sets, because using all this user friendly plug and play CPU sauce makes you weak! To be honest, I am part of an elite group who think the only REAL men left are the ones who go out every morning and mine up the silicon to build their own transistors from dust.
On a serious note, in my experience, the programming language is less relevant, but rather the experience is what makes you a better programmer. Working in other languages helps you get better, but in a way, understanding design patterns and program design, focusing on data encapsulation, modular design, inheritance systems, components, Model-View-Controller based application design etc; is far more important when it comes to keeping projects clean and succinct. Other languages do help with this understanding, as to an extent, OOP langauges enforce a form of data encapsulation by default.
Though recently, i've actually gone full-circle, I went through a phase of not liking GML for its limited features, but have grown to like it a lot again, though thats more to do with the way things like resource management and instance creation work in GM (Using object pools/having garbage collection in languages like C# is really awkward for games IMO).
At the end of the day, the langauge is what enables you to make your game, if you enforce reasonable habits with GM, then its as legitimate as any other language. You can write bad code and great code in any system, though I will say GMs simplicity and more relaxed nature can make it hard to learn good habits in the first place, so.. In short, learn other languages.
Real programmers learn to make use of the best tools available to them and more importantly use the most relevant tool for the task at hand