Which files the gcc compiler compile from?

The question is not that simple as it seems to be. GCC is not really a compiler; this is a key component of the GNU tool chain used to build software using a number of different languages. I personally used it for Ada, C and C++, but there are a lot more. The tool chain allows to create a new language in the form of a front-end and include in it a the build process.

Please see:
http://en.wikipedia.org/wiki/GCC_Compiler#Front-ends[^].

GCC is developed and primarily used Linux and other Unix-like OS, but also widely used on Windows.

So, the files used by GCC may vary. :-) Please see my comments to the question. There could be too many reasons in case your files do not compile. You need to address them one by one, usually. We would need way more detailed information to help you to compile the code. The problems of such compilation are very usual. It also depends on how the code is written, on the code quality and on the set of libraries used. Sometimes the code is very poor in portability, so the adaptation of such code to be built on Windows can be quite difficult and time-consuming task.

—SA