A large size prototype of a TPC with GEM amplification has been successfully built and operated. To obtain an active area larger than the one provided by the largest GEM foil on the market, 2 independent GEM towers have been hosted on a single pad plane minimising the dead space between the towers. This readout structure has been inserted in a test-bed available at CERN, inherited from the HARP-TPC installation at the PS East Hall, made by a cylindrical field-cage of 80 cm diameter and around 150 cm active length, all placed inside a solenoid magnet capable of 0.7 T. This setup is equipped with about 1500 readout channels. The initial choice for relatively large squared pads (8mm) is driven by the need to study the performances in view of a neutrino oscillation experiment, where the track density is low. A systematic study of the performances has been performed as a function of several parameters like the operating voltages of the GEM towers, the magnetic and the drift fields and the gas mixture. Special care has been taken in the study of the gas, with a large range of mixtures including Ar:CF4:iC4H10.