Even though FSF 6.0.0 is still in development, it's entered stage 3 which is unusally relatively stable for the Ada front-end. It has been placed in FreeBSD ports at lang/gcc6-aux, and the Ada framework can optionally use it instead of GNAT FSF 5.3.

It currently builds everything except OpenToken (caught by new code restriction / detection and needs code to be fixed) and Ironsides which fails during compilation with a STORAGE error, so an internal compiler error is suspected.

If you have FreeBSD or DragonFly and you want to testdrive GNAT 6, then add ADA_DEFAULT=6 to /etc/make.conf and rebuild everything from source.