  We present optical spectroscopy secured at the 10m Gran Telescopio Canarias
of the counterparts of 20 extragalactic gamma-ray sources detected by the Fermi
satellite. The observations allow us to investigate the nature of these sources
and to determine their redshift. We find that all optical counterparts have a
spectrum that is consistent with a BL Lac object nature. We are able to
determine the redshift for 11 objects and set spectroscopic redshift limits for
five targets. Only for four sources the optical spectrum is found featureless.
In the latter cases we can set lower limits on the redshift based on the
assumption that they are hosted by a typical massive elliptical galaxy whose
spectrum is diluted by the non thermal continuum. The observations allow us to
unveil the nature of these gamma-ray sources and provide a sanity check of a
tool to discover the counterparts of gamma-ray emitters/blazars based on their
multiwavelength emission.
