It's not bad at all, at least if you've made the conscious decision to write GNU C and not std C, and accept that non-gcc compilers (except maybe clang) may not be able to compile your code.

Unfortunately, though, I believe one of the -std=gnuXX variants is the default, so most people don't make that a conscious decision.

gnu89 is the default. gnu99 will be the default when C99 is fully implemented (from the manpage, search for /-std=$/)

C99 is still not fully implemented in GCC? What's missing?

There is a list at http://gcc.gnu.org/c99status.html