Elmer is an imager and spectrograph in the visible range that has been designed and managed within the GTC Project Office. Elmer will be installed at the telescope at the beginning of the commissioning phase. The observing modes of the instrument are: Imaging, Long Slit, Mask and Slit-less multi-object Spectroscopy, Fast Photometry and Fast short-slit Spectroscopy. The pupil elements are a set of conventional broad band and narrow band filters as well as a set of prisms, grisms and VPHs, that allow spectroscopy with resolving powers of 200, 1000 and 2500 between 365 and 1000nm. Elmer has been exhaustively tested and each of its observing modes has been fully characterized at the laboratory. This contribution summarizes the results of this Test Plan, showing the excellent performance of Elmer in both, Imaging and Spectroscopy modes that, together with the GTC, will lead to a powerful scientific return.