As I understand it, the rule is "if your language comes with a bignum implementation, you must use it, otherwise you may use GMP". I've been seriously tempted to suggest removing our bignum implementation for this reason (though I won't).
Python: http://benchmarksgame.alioth.debian.org/u64/program.php?test...
Ruby: http://benchmarksgame.alioth.debian.org/u64/benchmark.php?te...
When someone contributes a Rust pi-digits program that uses GMP it will be shown (as long as it uses the same algorithm).
We get to see both programs that use GMP and programs that don't.