It looks to be generally similar to C / C++
You should not have any great problems using that, if you are familiar with GML.
C, like many languages, needs all variables declaring before use, so the compiler knows what type of information is being stored and how much memory to allocate.
From a quick glance GML does not need that.
You also need to include the appropriate header files when using any common functions that are not directly built-in to the compiler, like
The stdxxx header has the definitions for the basic input / output functions, others are needed for more complex functions.
Any general "hello world" program will include those requirements, just copy that to start with.
C++ is really a set of extensions to C rather than a different language. Any normal C++ compiler can work with either "style" and you can freely mix the two, either procedural or object-oriented. The compiler produces the same code for equivalent operations in either style source, it does not care.
Use whichever you get on with best, or whichever fits what you are doing at the time.
(And "objects" are really just C "structs" - structures of variables - under a different name).