I have put patches which allow compiling gpc-20041218 with gcc-3.4.3. Note that the patch also contains support for qualified identifiers and a few fixes to structured value constructors.
The patch should work with all supported backends 2.8.1, 2.95.3, 3.2.3 3.3.x, 3.4.3 (for 2.8.1 you need an extra backend patch) -- it should just give you qualified identifiers support.
I would recommend the patch for AMD64, since gcc-3.3 on AMD64 has a number of problems which are fixed in gcc-3.4.3. Also, the patch works around packed array problem with gcc-3.3 on AMD64 (they did not work, with the patch the main problem is fixed).