In June 2011 we attempted to measure the first atmospheric transmission spectrum of an exoplanet with the GTC. We observed the hot Jupiter TrEs-2b during two different transit epochs in June 2011 using the long slit spectroscopic mode in Osiris. The technique consists of doing differential spectrophotometry by placing the planet host and another nearby star on a wide slit during a planetary transit. We then monitor both objects simultaneously for the duration of the event to obtain simultaneous transit coverage at wavelengths between ~ 0.5 and 0.9 microns. We present here the results of our first attempt with this new technique, which holds the potential of providing a 5-10 fold improvement on the efficiency of exoplanet primary and secondary eclipse observations with the GTC. Finally, we also present here a summary of the science done so far with observations from the ESO/GTC program 182.C.2018.