A while ago Victor Stinner has spend several weeks in porting PyLongs to
GMP:

http://mail.python.org/pipermail/python-3000/2007-September/010718.html
http://mail.python.org/pipermail/python-3000/2007-October/010755.html

Although his patch didn't give the speedup he hoped for, the patch might
be interesting someday in the future. He never submitted it to our bug
tracker. I'm posting it to conserve it for the future.

Since this keeps coming up, I think it would make sense to turn this
into an optional extension module.

PS. The licensing concerns are another reason not to use this for the
core long (or int in Py3k) type.

Why was the mpz module removed from Python 2.4 in the first place? 2.3
has  it.

I see three way to implement the option:

 * Let somebody implement a mpz type as a 3rd party extension.
 * Let somebody implement a mpt type and ship it with the Python core
 * Let somebody write a patch that replaces the built-in long type
implementation with a GMP based implementation (./configure
--with-gmp-integer)

I don't recall, but I suppose it had stopped working and nobody could be
found who wanted to maintain it.  Possibly the Python-unfriendly license
was also a reason.