GML is a distinct language developed for Game Maker. It bears some similarity to C and C++ on the surface, in terms of its syntax (keywords, grammar and structure), but it is otherwise unrelated to C++ -- it certainly is not a sub- or super-set of the C or C++ languages. GML differs from C(++) in a lot of ways... for example it does not require variable declaration, has a much simpler memory allocation model, and several additional extra language constructs (such as the "with" block).
You could write your GML code within Visual Studio, but it wouldn't work well. You might get syntax hilighting, but VS could not compile GML in general and using Game Maker itself to author GML is a far better idea since the tool and the language are tightly integrated.