The GEOS source tree on OSGeo ​Gogs instance (​https://git.osgeo.org/gogs/geos/geos) is a mirror of the official repository.

To get the source from !OSGeo Git Service, and start a configure:

git clone https://git.osgeo.org/gogs/geos/geos.git
cd geos
./autogen.sh
./configure

To fork GEOS to modify and test new features, just click on the Fork button on top of the geos page. You'll need to login using your OSGeo Userid (a link to register a new account will be found on the login page).