integer-gmp2: export `Word`-counterpart of gcdInt

It's trivial for `integer-gmp2` (#9281) to provide it, and it'll be
useful for a future 'Natural'-related commit, as well as providing a
`Word` optimised `gcd`-RULE.
Remove reference to `MIN_VERSION_integer_gmp2`

This is slipped in by accident as part of
c774b28f76ee4c220f7c1c9fd81585e0e3af0e8a (re #9281)