Satellite Navigation has become a keystone for the development of Europe and its citizens. It is then essential to provide adequate educational programmes so to ensure a prepared workforce for the GNSS sector in Europe. In this context, ESA has launched a complete Satellite Navigation Educational program, called EDUNAV, aiming at providing upto-date GNSS based educational material and educational tools. The GNSS-Lab (gLAB) Educational Software Tool is part of this ESA EDUNAV initiative. gLAB, developed under ESA Contract by the research group of Astronomy and Geomatics (gAGE) from the Universitat Politecnica de Catalunya (UPC), is an interactive educational multipurpose package to process and analyse GNSS data. gLAB performs precise modeling of GNSS observables (pseudorange and carrier phase) at the centimetre level, allowing both standalone GPS positioning and PPP. Every single error contributor may be assessed independently, which, in turn, provides a major educational benefit. gLAB is adapted to a variety of standard formats like RINEX-3.00, SP3, ANTEX and SINEX files, among others. Moreover, functionality is also included for GPS, Galileo and GLONASS, allowing performing some data analysis with real multi-constellation data. The gLAB software tool is quite flexible, able to run under Linux and Windows operating systems and is provided free of charge by ESA to universities and GNSS professionals.