  The Gemini Planet Imager (GPI) is a high contrast imaging instrument that
aims to detect and characterize extrasolar planets. GPI is being upgraded to
GPI 2.0, with several subsystems receiving a re-design to improve the
instrument's contrast. To enable observations on fainter targets and increase
stability on brighter ones, one of the upgrades is to the adaptive optics
system. The current Shack-Hartmann wavefront sensor (WFS) is being replaced by
a pyramid WFS with an low-noise electron multiplying CCD (EMCCD). EMCCDs are
detectors capable of counting single photon events at high speed and high
sensitivity. In this work, we characterize the performance of the HN\"u 240
EMCCD from N\"uv\"u Cameras, which was custom-built for GPI 2.0. The HN\"u 240
EMCCD's characteristics make it well suited for extreme AO: it has low dark
current ($<$ 0.01 e-/pix/fr), low readout noise (0.1 e-/pix/fr at a gain of
5000), high quantum efficiency ( 90% at wavelengths from 600-800 nm; 70% from
800-900 nm), and fast readout (up to 3000 fps full frame). Here we present test
results on the EMCCD's noise contributors, such as the readout noise,
pixel-to-pixel variability and CCD bias. We also tested the linearity and EM
gain calibration of the detector. All camera tests were conducted before its
integration into the GPI 2.0 PWFS system.
