pgAdmin is available subject to the terms of the licence. pgAdmin is developed using a number of GIT repositories to hold the source code. Access to the repository is available through a web interface or using your favourite GIT client.
To checkout the source from the command line and get started, you might use commands such as shown below on a Linux or Unix system. For further details, please see the source code build instructions.
git clone git://git.postgresql.org/git/pgadmin4.git