I have tested build with gcc-3.4.6, gcc-4.1.2, gcc-4.2.4 and gcc-4.3.5.
Before build copy or symlink the p subdirectry of gpc into gcc-x.y.z/gcc
subdirectory of gcc tree (that is you should have directory called
gcc-x.y.z/gcc/p).  To build with gcc-3.4.6 (or earlier) apply backend
patch and proceed as with previous gpc versions.
To build with gcc-4.y copy the libgpc subdirectory into toplevel gcc
directory (that is into gcc-4.y.z) and apply the patch backend patch
from _toplevel_ directory (that is inside gcc-4.x.y directory) using
command:

patch -p1 < gcc/p/gcc-4.y.z.diff

Then configure and build.  Using gcc-4.3.5 you need to give 'configure' 
the '--disable-mapped-location' argument.

Known problems:

1) with gcc-4.y.z testsuite will fail if run immediately after build.
Either install before running testsuite or copy 'libgpc.a' from libgpc
build directory to gcc subdirectory and copy rts-confing.inc from libgpc
build directory to gcc/p/rts subdirectory (you need to create rts
subdirectiry)
2) I did not try to build shared libgpc -- it is probably broken.
You would need to fix Makefiles to build it.
3) -Wparenthesis does not work with gcc-4.3.z
4) Utilities (binobj and gpidump) are not build with gcc-4.y.z