libp6gui is part of libpumpkin (http://libpumpkin.g76r.eu/) which is a free software collection of tools for using Qt/C++ on the client side, as a complement of libp6core
This software is made availlable to you under the terms of the GNU Affero General Public License version 3, see AGPL-3.0.txt file content.
This software can be build with Qt's qmake build tool or a Qt-aware IDE like Qt Creator.