GKextCPy is a python extension building the wrapper from C++ to Python and necesssary to compile the graphkernels package.
If you install the graphkernels package this extension is automatically installed for you. However, you can install it independently via pip:
pip install GKextCPy
The interface files which allow the wrapping of eigen and numpy (files numpy.i and eigen.i) are imported from the PyMesh project at https://github.com/qnzhou/PyMesh/tree/master/eigen_wrapper