After its first light in November 2013 the Gemini Planet Imager (GPI) is ending its on-sky commissioning phase and about to be offered to the science community to start his exoplanet hunting campaign. This instrument consists of an Extreme Adaptive Optics (XAO) system combined to a Calibration (CAL) unit providing very high quality wavefronts to feed an Integral Field Spectrograph imager working in the near-infrared range (standard YJHK bands); an Apodized-Pupil Lyot Coronagraph (APLC) is tightly integrated within these 3 sub-systems and is the key element masking out the starlight to reveal orbiting exo-objects.

The CAL unit was especially designed by the Jet Propulsion Laboratory to control AO-unseen slowly varying wavefront errors at the focal plane of the APLC by the means of two wavefront sensors: a 7x7 low order Shack-Hartmann (LOWFS) and Mach-Zehnder type interferometer for mid-order spatial frequencies. Original design goals aimed for sensing and correcting these non-common path aberrations on a level of a few nanometers which is extremely challenging in a telescope environment. 

I also present here the recent investigations and tests carried on the LOWFS with a view to commission the offload of non-common path low-order aberrations to the XAO system.