One of the constants of GNU products is a huge number of features. gcc has an incredible number of extensions available. They can help improve your code, but they can also destroy portability if you target a platform that does not have gcc or you are not using gcc on Mac OS X.
Here are a handful of interesting extensions:
long long and unsigned long long: quadword (i.e., 64-bit) integers are treated as first class citizens. You can perform basic math (+,-,*,/), modular arithmetic, and bitwise operations with them.
Complex numbers: in C++ you can create complex numbers as a first class type, but gcc C has them built in.
Variable length automatic arrays: like you can use in C++, declaring the size of a stack-based array at runtime: ...