Thanks to their exceptional brightness gamma-ray burst (GRB) afterglows can be used as extra-galactic background sources capable of unveiling the properties of the universe at different redshifts. The class of long GRBs (LGRBs) is associated with the collapse of the most massive stars, making them a tool to investigate star formation in the early Universe. The goals of this project are: (a) to study the properties of a complete sample of LGRB host galaxies; (b) to combine up to the highest redshifts the properties of the absorbing gas in the inter-stellar medium (ISM) of GRB host galaxies derived from the afterglow spectroscopy with those of the ionized gas derived from the host galaxy spectroscopy. The results will 1. allow a robust use of LGRBs as SFR tracers in the Universe; 2. put indirect constraints on the GRB progenitor star models; 3. allow the characterization up to the highest redshift of a population of low-mass star forming galaxies forming the bulk of the high redshift galaxy population; 4. bring unique information on the link between the properties of the ISM and star formation activity up to very high redshift. The project presented here gives the opportunity to the Scientific Coordinator to build her team and to develop a strong group in France for the use of GRBs as probes of the high-redshift Universe. Bringing to new and unique results, this project will boost the French scientific activity in this field and will enhance the scientific return of the SVOM mission.