The Adaptive Optics system of the 10-m class Gran Telescopio Canarias (GTCAO) is completing the acceptance tests in the laboratory at the Instituto de Astrofísica de Canarias, to be ready for its integration in the telescope at Roque de Los Muchachos Observatory (ORM) in 2023. The AO system has been designed with robustness and operability as its key characteristics, and will be a facility of GTC. It features a single deformable mirror (DM) with 373 actuators, conjugated to the telescope pupil, and a Shack-Hartmann wavefront sensor (WFS) with 312 subapertures, using an OCAM2 camera. The expected performance of the GTCAO system working on average atmospheric conditions and bright NGS is 65% Strehl Ratio in K-band. In this paper we present the characterization of the system and the tests that have been performed for its acceptance at the laboratory. A series of calibrations are required and methodically run to achieve its ultimate performance: flatten the DM, acquire the Interaction Matrix and the reference slopes, correct the non-common path aberrations etc. The WFS requires additional calibrations, to compensate the pupil displacement in all the 2 arcminutes patrol field of view, and to correct the atmospheric dispersion in the visible. Close loop gains and sampling frequency are adapted to the changing conditions, and lookup tables are created for that purpose. The compliance with the system specifications has been verified. After verification of the final software functionalities for telescope operation, the system will be shipped to the ORM, to be installed and aligned on the Nasmyth platform of the GTC, and to be integrated with the telescope control system.