Looks like one at least existing binding. The other one, mentioned perhaps more often (AdaGMP: http://www.ii.uib.no/~gisle/adagmp/), does not seem to be up anymore. According to the copyright year these bindings were not touched since 1999, so this is likely to be against a rather old gmp version. Nonetheless it might be usefull to someone. Still, expect that much cleanup or updating is necessary.. George
This page seems to have more references to libs dealing with large numbers: http://www.chez.com/bignumber/index.html
Another binding surfaced on comp.lang.ada. Looks like this one is the most "modern" and complete and has an author who actually is not MIA: http://terratope.tigris.org/source/browse/terratope/trunk/cy/gmp/
Not exactly the same thing, but related - "The Big_Number package". Reportedly not as efficient as gmp though. http://www.chez.com/bignumber/info.htm