The GCC extensions are just that - they're not part of the standard. You can claim 100% standards conformance without implementing a single one of them.
That said, in practice a large amount of C code out there is not pure standard C so if you want to be able to compile something "interesting" like e.g. a Linux kernel you'll need to implement some.
That said, in practice a large amount of C code out there is not pure standard C so if you want to be able to compile something "interesting" like e.g. a Linux kernel you'll need to implement some.