To use, clone the repo into a temporary directory, using:
git clone https://github.com/kalmi/gedit-themes.git
Then copy them into a styles folder in the gEdit directory, using the install script from the repo:
cd gedit-themes ./install.sh
To use them, open gEdit, then go to Edit -> Preferences -> Fonts and Colours. The themes should now be available in the Colour Scheme scroll box.