One option is to install MacPorts and install the
sudo port install gcc46
Another option is to download the source code and build it as follows:
tar xzvf gcc-4.6.0.tar.gz cd gcc-4.6.0 ./configure make
Note that GCC 4.6.0 requires as prerequisites GMP 4.2+, MPFR 2.3.1+, and MPC 0.8.0+. If
./configure fails, it's probably because you're missing one of these (though it should give you a helpful error message in any case).
Building will take a while—likely several hours, depending on your hardware.