The Gemini Planet Imager (GPI) is a high-contrast instrument specially designed for direct imaging and spectroscopy of exoplanets and debris disks. GPI can also operate as a dual-channel integral field polarimeter. The instrument primarily operates in a coronagraphic mode which poses an obstacle for traditional photometric calibrations since the majority of on-axis starlight is blocked. To enable accurate photometry relative to the occulted central star, a diffractive grid in a pupil plane is used to create a set of faint copies, named satellite spots, of the occulted star at specified locations and relative intensities in the field of view. We describe the method we developed to perform the photometric calibration of coronagraphic observations in polarimetry mode using these fiducial satellite spots. With the currently available data, we constrain the calibration uncertainty to be <13%, but the actual calibration uncertainty is likely to be lower. We develop the associated calibration scripts in the GPI Data Reduction Pipeline, which is available to the public. For testing, we use it to photometrically calibrate the HD 19467 B and β Pic b data sets taken in the H-band polarimetry mode. We measure the calibrated flux of HD 19467 B and β Pic b to be 0:078±0:011 mJy and 4:87±0:73 mJy, both agreeing with other measurements found in the literature. Finally, we explore an alternative method which performs the calibration by scaling the photometry in polarimetry mode to the photometrically calibrated response in spectroscopy mode. By comparing the reduced observations in raw units, we find that observations in polarimetry mode are 1:03 0:01 brighter than those in spectroscopy mode.