You can install GCC 4.9 by building it from ports with
cd /usr/port/lang/gcc49; make install clean
or if you have
portmaster -DHB lang/gcc49
or if you prefer packages with
pkg install lang/gcc49
If you change
lang/gcc you will install the most recent stable version of GCC currently this is GCC 4.7.
When you want to build all your ports with GCC instead of Clang you have to edit
/etc/make.conf and add
.if !empty(.CURDIR:M/usr/ports/*) && exists(/usr/local/bin/gcc49)
/etc/libmap.conf and add there
Here is an article from FreeBSD.org which explains this in more detail.
But you don't really need GCC for the ports, all ports that already depend on GCC will use GCC.
g++ will be installed with all GCC ports.