Get Xcode from the App Store and install it. You also need to install the command line tools, do this with the command:

$ xcode-select --install

Install Homebrew:

$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"
$ echo 'export PATH=/usr/local/bin:$PATH' >> ~/.bash_profile

Install ASE and GPAW dependencies:

$ brew install python
$ brew install gcc
$ brew install libxc
$ brew install open-mpi
$ brew install fftw

Install pip:

$ sudo easy_install pip

Install required Python packages:

$ pip install numpy scipy matplotlib

Install and test ASE:

$ pip install --upgrade --user ase
$ python -m ase test

Use this siteconfig.py file:

fftw = True
libraries = ['xc', 'fftw3']

Install GPAW:

$ pip install --upgrade --user gpaw

Install GPAW setups:

$ gpaw install-data


Alternative solution if the above command fails:

$ curl -O https://wiki.fysik.dtu.dk/gpaw-files/gpaw-setups-0.9.20000.tar.gz
$ tar -xf gpaw-setups-0.9.20000.tar
$ echo 'export GPAW_SETUP_PATH=~/gpaw-setups-0.9.20000' >> ~/.bash_profile

Test GPAW:

$ gpaw test
$ gpaw -P 4 test