First, please note that having gfortran testresults for one platform only means that "some version of the compiler was able to correctly compile GMP & MPFR", not that "GCC trunk is able to correctly compile GMP & MPFR". Nevertheless:
* i386-unknown-freebsd (alpha-unknown-freebsd5.4) http://gcc.gnu.org/ml/gcc-testresults/2005-07/msg01072.html
I can confirm that GMP & MPFR build fine and run their testsuite cleanly on i386-unknown-freebsd{4.7,5.4,5.11}. I know Steve Kargl uses this on x86_64-unknown-freebsd6.1 and it works fine. As for other BSDs, I can confirm that GMP & MPFR build and run their testsuite fine on i386-unknown-netbsd2.0.2, i386-unknown-openbsd{3.7,3.8}.
* sparc-sun-solaris2.10 http://gcc.gnu.org/ml/gcc-testresults/2006-01/msg01273.html
I can also confirm that GMP & MPFR build and run fine on i386-unknown-solaris2.9.
* ia64-unknown-linux-gnu http://gcc.gnu.org/ml/gcc-testresults/2006-03/msg01826.html
I my experience, GMP & MPFR do also work fine on ia64-hpux, but that requires a recent GMP version (>= 4.2.0, if I remember correctly; before that, the configury didn't work well for ia64-hpux). Also, I have had GMP & MPFR building and testing fine on alpha-linux for some time. Finally, as S/390 GNU/Linux seems to be considered for inclusion into this list, it is worthy to note that gfortran is built & regtested on this platform and variants regularly. FX