The GMP project uses Mercurial for its revision control. Mercurial is a distributed SCM, where your checkout is just another repository. Where the central repository lives is just an agreement between developers, not something Mercurial enforces.

Once you have Mercurial installed, do

hg clone http://gmplib.org:8000/gmp TARGDIR

for the unstable development version, or

hg clone http://gmplib.org:8000/gmp-5.0 TARGDIR

for the current patch level of GMP 5.0, or

hg clone http://gmplib.org:8000/gmp-4.3 TARGDIR

for the current patch level of GMP 4.3.

See also the Mercurial quickstart guide.

Once you have your own repository, you need to generate some build files. Perhaps the script .bootstrap works out-of-the box; if it doesn't, you need to install automake-1.8 and autoconf (at least version 2.59) and then try .bootstrap again.

Now you should be able to build GMP as usually, i.e., with

configure OPTIONS
make
make check