The PostGIS source tree on !GitLab (https://gitlab.com/postgis/postgis) is a mirror of the Subversion repository (http://svn.osgeo.org/postgis/).

To get the source from !GitLab, and start a configure:
{{{
git clone https://gitlab.com/postgis/postgis.git
cd postgis
./autogen.sh
./configure
}}}
To fork PostGIS to !GitLab to modify and test new features, follow the guideline [http://doc.gitlab.com/ee/workflow/forking_workflow.html Project Forking Workflow].