GCC 6.5 Released To End Out The GCC6 Series
GCC 6.5 fixes more than 250 bugs/regressions since the earlier GCC 6.4 release. Users are encouraged to upgrade to GCC 6.5 if not able to upgrade yet to GCC7 or GCC8 stable series.
The brief GCC 6.5 release announcement can be read on their mailing list.
GCC 9.0 meanwhile is the current development version of the compiler that will premiere as GCC 9.1 stable in early 2019. GCC 9 feature development will be ending in November to move onto a bug-fixing and documentation focus.