Download gfortran source code
As gfortran is a new compiler still under development, chances are that the official releases are lagging a bit behind. The recommended way to enjoy the latest gfortran compiler is to build it from the subversion tree (see below).
Subversion repository -- development code
You can access the GCC source tree under development from the subversion repository.
If you're reluctant to use subversion, weekly snapshots of the source tree can be found on the GCC FTP site.
Configuration and compiling
See the compiler build instructions, but in a nutshell you do: create a new directory, e.g. ../gcc-build, go there and run ../gcc/configure --prefix=$HOME/gcc-trunk --enable-languages=c,fortran. You may want to add more languages, --enable-checking=release (esp. useful for compile-time tests as it disables some consistency checks in the compile), --disable-bootstrap (builds faster, but disables some consistency checks). After make and make install you have built your own GCC.
Make sure that you have the MPFR installed, which is needed to compile the GCC.