GmtPy is implemented in a single source file gmtpy.py, which you may just place into the directory, where you want to use it. However I recommend to install it properly, as described below.
Ensure that the $GMTHOME environment variable is set properly.
GmtPy is hosted at GitHub, so the simplest way to download is to use git:
cd ~/src/ # or wherever you keep your source packages
git clone git://github.com/emolch/gmtpy.git gmtpy
Alternatively, you may download GmtPy as a tar archive, but updating is easier with the method described above.
To install GmtPy system wide (usually somewhere under /usr/local), simply run (from within GmtPy’s source directory):
sudo python setup.py install
It is also possible to install to a custom location. To do so, use the --prefix option of setup.py and adjust the environment variable $PYTHONPATH (see distutils documentation for details).
If you used git to download GmtPy, and you would later want to update, use the following commands (from within GmtPy’s source directory):
git pull origin master
sudo python setup.py install