Transmission spectroscopy has been a powerful tool to decipher the atmospheric compositions of transiting exoplanets. The recent HST survey has shown that clouds might be common at the terminator region which would mute the expected spectral signatures to some extent. With potential clouds complicating data interpretation, there is still a long way to go before exoplanet atmospheres can be well understood. We are currently conducting a transmission spectroscopy survey on a large number of exoplanets using the long-slit mode of the OSIRIS instrument at the 10.4-m Gran Telescopio Canarias (GTC). Targets, with types from super Earths to Neptune-sized and Jupiter-sized planets, are strategically selected. This survey will not only improve our understanding of exoplanet atmospheres' diversity, but also provide a set of candidates that are relatively cloudless thus favorable for future detailed characterization. I will show how we use a transiting white dwarf to validate our method with GTC/OSIRIS. I will discuss the impact of instrumental systematics and the lessons learnt for future observations. I will also present some early science results from our survey.