"C99 is substantially completely supported as of GCC 4.5 (with -std=c99 -pedantic-errors used; -fextended-identifiers also needed to enable extended identifiers before GCC 5), modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G)." [1]

Sounds fully supported to me, for all practical purposes.

"Default standard is now GNU11" [2]

What do you know that I don't?

[1] https://gcc.gnu.org/c99status.html

[2] https://gcc.gnu.org/gcc-5/porting_to.html