The upcoming FSF GCC5 has not even produced a release candidate yet, but that didn't prevent GNAT 5 from being put into ports, making it available to FreeBSD and DragonFly BSD. After some adjustments to the other ports, it builds everything except GtkAda version 3.

The problem with GtkAda is that it uses invalid Ada according to GNAT 5, but Adacore has migrated from Subversion version control to Git, and none of Adacore's respositories are publically available. Supposedly they will be at some point in the future, but it is not a priority, and as a result, the GtkAda fix is not available either. Hopefully the repositories will be mirrored on GitHub in the future, but there's no timeline for that.

The new port is located at /usr/ports/lang/gcc5-aux, and it will install at /usr/local/gcc5-aux/bin. To build all ports with it, you'd have to put "ADA_DEFAULT=5" in /etc/make.conf, but I do not recommend that you do this unless you are testing the port.