Edit: updated to reflect updated question.
Clang and GCC shouldn't have any noticeable differences to most code aside from some default warnings IIRC. what is your specific problem? Is it that you are compiling a binary on a mac and your teacher is on windows? your teacher needs to tell you the errors if you want to fix them.
If you do want to use GCC with xcode you need to download and install it separately.
I dont know what CGG is, but you can change the setting by selecting your project/target in xcode then going to "build settings" then change the "c language dialect" setting.
AFIK there is no C90 do you mean C89? why in the world does your teacher insist on using that?! we are up to C11 now...