In the past years ESO has built together with industry a transportable Laser Guide Star Unit (WLGSU) to test the laser technology while supporting some strategic experiments for the LGS-AO technologies. Since July 2016, the WLGSU has been installed at the Observatorio del Roque de Los Muchachos (ORM) in La Palma, next to the William Herschel Telescope (WHT), to perform different field experiments and studies related to the LGS technologies for Adaptive Optics. To study the photometric and spectral contamination from the scattering near the laser beam along the uplink at different heights, including the LGS itself, four observing runs, each of four hours, have been granted under Program GTC3-16ITP using the OSIRIS spectro-imager at the 10.4m Gran Telescopio CANARIAS (GTC). We report preliminary results of the first two runs on the photometric and spectroscopic measurements and on the observed Raman emission of atmospheric molecules along the uplink beam.