  We report on optical spectroscopy obtained at the 10.4m Gran Telescopio
Canarias of 28 Fermi gamma-ray sources that completes the study of a sample of
60 targets of unidentified objects for which the detection of an X-ray and/or
radio source inside the 3FGL error box is available. The observations are aimed
to characterize the nature and measure the redshift of these sources. For all
optical counterparts, the observations allow us to establish their AGN nature.
In particular, we found 24 BL Lac objects, one QSO, one NLSy1 and two objects
showing spectral features typical of Seyfert 2 galaxies. For most of them, we
determine a spectroscopic redshift, while for five we can set lower limits
based on the lack of stellar features from the host galaxy. The global
properties of the full sample are briefly discussed.
